Chemistry Review- Third Quarter Benchmark (100 point test grade; Tuesday, April 6th; this review is due at 8am April 6th)
- What happens to the particles of a substance when it is heated? Cooled?
When a substance is heated, the particles move apart. When a substance is cooled, the particles move closer together and the substance becomes more rigid.
- If a gas occupies 3.8 L at a pressure of 2.71 atm, what would the pressure be if the volume changes to 1.47 L? (Temp. and amt. of gas constant)
V1=3.8 L; P1=2.71 atm
V2=1.47L; P2=? Atm
Since all units are standard (L, atm), we do not need to convert the units before proceeding.
(P1V1)/T1=(P2V2)/T2
T goes away (not mentioned in problem)
P1V1=P2V2
Isolate for P2: P2= (P1V1)/V2
P2= (2.71 atm x 3.8 L)/(1.47 L)= 7.0 atm (L cancelled and it’s 2 sig figs)
- What is mass number? Atomic number?
The mass number is the amount of protons and neutrons in an atom’s nucleus. This is found below the element symbol on the periodic table.
The atomic number is the number of protons in the nucleus (also number of electrons in a neutral atom). It is found above the element symbol.
- Define the terms exothermic and endothermic.
In an exothermic process, heat is released from the system to the surroundings. For example, when a cup of hot coffee (system) cools, it transfers it’s heat to the room around it (surroundings). The direction if heat transfer is out of the system.
In an endothermic process, heat is absorbed by the system from it’s surroundings. The direction of heat transfer is into the system. For example, when you heat a pot of water on the stove, the cold water (system) warms up because it receives heat from the stove (surroundings).
- What is happening inside a balloon when it is inflated? What happens when you put the balloon in the freezer? Why?
Many things happen inside a balloon as it is filled with gas (inflated). The amount of gas particles (n) are increasing. As you add more gas, there are more molecules to hit the sides of the container (balloon).
Since there are more molecules hitting the side of the container, the pressure against the container increases. As the pressure inside the container increases, the volume of the container increases (this would not happen if the container was not flexible like a balloon). Yes, pressure and volume are typically inversely proportional. This is because if you increase the pressure on the outside of a gas, it will squish the gas molecules closer together. Inflating a balloon (or a tire or anything flexible) can happen because the pressure inside the container is greater than the pressure outside the container. In most problems, pressure will be referring to the pressure outside the container, since pressure is typically applied by the atmosphere or a piston, which are both external.
Once the container is filled with more particles and more particles are hitting the sides, more kinetic energy is generated, thus increasing the temperature inside the balloon (slightly).
When you put a balloon into a freezer, the kinetic energy is decreased (as a result of decreasing temperature); the molecules slow down and do not bump against each other or the side of the balloon as much. Since there are fewer molecules bumping against the sides of the container, the pressure inside the balloon decreases. As a result of this decrease in pressure, the volume decreases.
- If 17.4 g of NaOH is dissolved to water to make 1.48 L of solution, what is the molarity of the solution?
Molarity (M)= moles/volume= moles/L
First we need to convert grams to moles:
17.4 g NaOH x (1 mole NaOH/40 g NaOH)= 0.435 moles
Molarity (M)= moles/L= 0.435 moles/ 1.48 L= 0.294 M
- How can a person speed up the process of dissolving sugar in water?
One way to speed the dissolving process would be to crush the sugar into smaller particles (like powdered sugar). This will increase the surface area. Increasing the surface area speeds the dissolving process because dissolving can only happen on the surface of the substance.
Another way to speed up the process would be to heat the solvent. Adding heat would increase the temperature of the solvent, thus increasing the kinetic energy of the solvent particles. Kinetic energy = energy of motion. Thus, more heat = more movement. More movement of the solvent particles means that solvent will bump into solute at a faster rate.
Another way to speed the process would be to stir the sugar and water. Stirring moves the dissolved sugar away from the surface of the un-dissolved sugar and replaces this with fresh solvent to dissolve the sugar. (very similar to what happens when you add kinetic energy through heat)
- How does gas create pressure inside a tire or on another surface?
The particles in a gas bump into a surface. When they bump into the surface, they exert pressure on it. (see also question 5)
- Why won’t olive oil dissolve in water?
Oil is not soluble in water. Like dissolves like. That means that polar solvents dissolve polar substances and nonpolar solvents dissolve nonpolar substances. Water is polar and oil is not. Thus water will not dissolve oil. Water will dissolve something like alcohol because both alcohol and water are polar.
- A gas has a volume of 1.49 L at a temperature of 34.75 degrees Celsius. What would the volume be at 78.41 degrees Celsius? (pressure & amt. of gas constant)
V1= 1.49 L, T1= 34.75 degrees Celsius
V2= ? L, T2= 78.41 degrees Celsius
We need to convert from Celsius to Kelvins before proceeding.
T1= 34.75 + 273= 307.75 K
T2= 78.41 + 273= 351.41 K
(P1V1)/T1= (P2V2)/T2
Get rid of P
V1/T1= V2/T2
Isolate V2: V2= (V1/T1)T2
V2= (1.49 L/307.75 K)(351.41 K)= 1.70 L
- What is a Bronsted-Lowry base? Acid?
The Bronsted-Lowry theory is concerned with H+ (protons). A Bronsted-Lowry base accepts a proton (H+). A Bronsted-Lowry acid donates a proton (H+).
This differs from the Arrhenius theory in which acids donate H+ and bases donate OH-. This was discovered because Arrhenius noticed that these substances will transmit electric current (are electrolytes).
This also differs from the Lewis theory in which bases donate electron pairs and acids accept electron pairs.
- What is a strong acid? Weak Acid?
A strong acid dissociates completely in aqueous solution. It is a strong electrolyte. Some examples include hydrochloric acid, sulfuric acid, and perchloric acid.
A weak acid dissociates only partially in solution. In fact, most of it does not dissociate. These acids are only weakly electrolytic. The best example is acetic acid.
- What is diffusion? Which would diffuse faster, a big particle or a small particle? How might temperature affect the rate of diffusion?
Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. For example, when you spray perfume, it is highly concentrated on the skin where you sprayed it, but the evaporated particles move from that highly concentrated area to fill the room.
Smaller, lighter particles will diffuse faster. For example, rubbing crushed flowers on your skin for perfume would not fill the room with scent as much as rubbing the same amount of perfume on your skin. The crushed flower particles are heavier and will take longer to fill the room.
- What volume is occupied by 8.47 g of hydrogen gas at 84.7 degrees Celsius and 1.04 atm?
The combined gas law won’t work here. You only have one condition and nothing changes.
PV=nRT will work. (n= moles and R= 0.08026 L atm/K mol)
P= 1.04 atm, V= ? L, n= OOPS we need to convert from grams to moles!, T= 84.7 degrees Celcius + 273= 357.7 K
n: Hydrogen gas is diatomic!
8.47 g H2 x (1 mol H2/ 2.02 g H2)= 4.193069307 mol H2
Isolate V: V= (nRT)/P= (4.193069307 mol H2 x 0.08206 L atm/ K mol x 357.7 K)/ 1.04 atm= 118 L
- Is a solution with a pH of 8.49 acidic or basic?
Acidic: pH<7
Neutral: pH= 7
Basic: pH>7
pH 8.49 is basic
- What is STP?
STP stands for conditions of standard temperature and pressure. Standard temperature is 273 K (or 0 Celsius). Standard pressure is 1 atm (or 760 mmHg, or 760 torr, or 101.3 kPa).
- How do you convert from Celsius to Kelvin?
Kelvin= Celsius + 273
- How do you convert from atmospheres to mm Hg? From atmospheres to kPa?
1 atm= 760 mmHg
1 atm= 101.3 kPa
- What volume is occupied by 56.75 g of oxygen gas at STP?
You can do this problem two different ways. The first choice would be to use the ideal gas law, PV=nRT:
P= 1 atm, V= ? L, n= 56.76 g O2 x (1 mol O2/ 32 g O2)= 1.77375 mol O2,
R= 0.08206 L atm/ K mol, T= 273 K
V= (nRT)/P= (1.77375 mol O2 x 0.08206 L atm/K mol x 273 K)/ 1 atm= 39.74 L
The second choice would be to use the conversion factor 22.4 L/mol. This will work because the ideal gas is at STP. YOU CANNOT USE THIS CONVERSION FACTOR UNLESS THE GAS IS AT STP. This method is faster:
56.76 g O2 x (1 mol O2/ 32 g O2) x (22.4 L O2/ 1 mol O2)= 39.73 L
- Is a solution in which [OH-] >[H+] acidic or basic?
When a solution is acidic, the H+ overpowers the OH-. When a solution is neutral, the H+ and OH- are exactly equal. In a basic solution, OH- overpowers H+.
So when [OH-]>[H+], the solution is basic.
- How much energy is required to raise 74.58 g of iron from 37.44 degrees Celsius to 98.39 degrees Celsius? (specific heat of iron p. 329)
Since the phase does not change and we are heating or cooling a substance within the same phase, we use Q= mc∆T (c=s, so don’t worry if you use s). Q is heat energy in Joules, c is specific heat, m is mass, and ∆T is the difference between the final and initial temperatures.
c for iron is 0.45 J/g °C. *it is very important that all your units in the problem match the value you are given for c.
∆T= Tfinal– Tinitial= 98.39 °C- 37.44 °C= 60.95 °C
Q=mc∆T= 74.58 g x 0.45 J/g °C x 60.95 °C= 2045 J
- If 15.71 g of oxygen gas has a volume of 8.14 L, What is the volume of 48.39 g?
In this problem, they did not give us anything about pressure or temperature, so we cannot use the combined gas law or the ideal gas law. The only law that will work is one that involves only volume and moles of gas (because we can convert from grams to moles). Enter Avogadro! Avogadro’s law: (V1/n1)=(V2/n2); Avogadro's Law states that for a gas at constant temperature and pressure the volume is directly proportional to the number of moles of gas; as the number of particles increases, the volume also increases
We could convert grams to moles, but this conversion factor will end up on both sides of the equation and will cancel out, thus we can just use grams.
Isolate V2: V2= (V1/n1)n2
V2= (8.14 L/ 15.71 g) 48.39 g= 25.1 L
*If you convert to moles first you will get the same answer.
- When ice changes to liquid water, is heat energy absorbed or released?
The terms exothermic and endothermic refer to the flow of heat energy. COLD DOES NOT MOVE, ONLY HEAT. When energy is absorbed by the system, the process is endothermic. When energy is released by the system, the process is exothermic. So, when cold ice melts into liquid water, it has to gain some heat. The ice absorbs heat energy, which allows its particles to start moving more and that makes it a liquid. (This process is endothermic)
- How much energy is required to change 28.45 g of water from 20.05 degrees Celsius to 87.48 degrees Celsius?
Since the phase does not change and we are heating or cooling a substance within the same phase, we use Q= mc∆T (c=s, so don’t worry if you use s). Q is heat energy in Joules, c is specific heat, m is mass, and ∆T is the difference between the final and initial temperatures.
c for water is 4.184 J/g °C. *it is very important that all your units in the problem match the value you are given for c.
∆T= Tfinal – Tinitial= 87.48 °C- 20.05 °C= 67.43 °C
Q=mc∆T= 28.45 g x 4.184 J/g °C x 67.43 °C= 8027 J
- If hydrogen gas occupies a volume of 2.49 L at 3.41 atm, what would the volume be at 1.37 atm? (Temp. and amt. of gas constant)
Start with the combined gas law and eliminate what you don’t need.
(P1V1)/T1=(P2V2)/T2
P1= 3.41 atm, V1=2.49 L
P2= 1.37 atm, V2= ? L
Eliminate T
P1V1=P2V2
Isolate V2: V2= (P1V1)/P2= (3.41 atm x 2.49 l)/1.37 atm= 6.20 L
- When ice forms from liquid water is heat energy absorbed or released?
This is the opposite of #23. In this case, water has to release heat in order to become ice. When the water releases heat, its molecules slow down to enter the solid phase. *Your freezer serves to take away heat. Don’t think of it as adding cold.
- Distinguish between a solvent and a solute.
A solvent is the substance that does the dissolving. It is usually the substance present in the greatest quantity. Everything is dissolved in the solvent (provided it is soluble).
The solute is the part of the solution that is being dissolved. There is typically less of the solute.
For example, in chocolate milk, milk is the solvent and chocolate syrup is the solute.
- Define the terms unsaturated, saturated, and supersaturated.
An unsaturated solution can hold more solute. This usually happens when a solute has not finished dissolving yet (solute was just added and is still sitting on the bottom of the container).
A saturated solution contains the maximum amount of solute that the solvent can hold at that temperature. A saturated solution will have a few particles left on the bottom that could not dissolve.
A supersaturated solution contains more solute than that solvent should be able to hold. Typically, a saturated solution is formed by heating a solvent so that all the extra particles on the bottom will dissolve (as compared with a saturated solution in which some particles are left on the bottom). Supersaturated solutions typically do not have any particles leftover on the bottom. However, these solutions are temporary. Once they cool down, they lose some of their dissolving power and some of the solute will begin to fall out of the solution, sometimes as crystals.
- If 74.81 g of NaOH is dissolved in 741.2 g of water, what is the percent composition of NaOH?
% composition= (mass of solute/mass of solution) x 100%
*make sure that you add both the mass of the solute and the mass of the solvent when calculating the mass of the solution
% composition NaOH= 74.81 g NaOH/(74.81 g NaOH + 741.2 g water)x100%= 9.168% NaOH
- What color does litmus paper change to in acid? Base?
Litmus paper turns red in acid and blue in base.
Red=Ac-ed, basic blue
- If 19.41 g of KOH are dissolved in 247.5 g of water, what is the concentration in parts per million (ppm)?
Parts per million (PPM). Parts per million works like percent by mass, but is more convenient when there is only a small amount of solute present. PPM is defined as the mass of the component in solution divided by the total mass of the solution multiplied by 106 (one million):
A solution with a concentration of 1 ppm has 1 gram of substance for every million grams of solution. Because the density of water is 1 g per mL and we are adding such a tiny amount of solute, the density of a solution at such a low concentration is approximately 1 g per mL. Therefore, in general, one ppm implies one milligram of solute per liter of solution.
Finally, recognize that one percent = 10,000 ppm. Therefore, something that has a concentration of 300 ppm could also be said to have a concentration of (300 ppm)/(10,000 ppm/percent) = 0.03% percent by mass. (Obtained from:
**Note that this is very similar to calculating % composition (percent by mass). In this case you are multiplying by 106 instead of 100%.
PPM= [19.41 g KOH/(19.41 g KOH + 247.5 g water)]x106=
0.07272 x 106 ppm= 7.272 x 104 ppm= 72720 ppm