LAB 7
MASS CHANGES IN CHEMICAL REACTIONS
Objectives 1a, 8, 9
lab objectives 1-4
BACKGROUND: When a chemical reaction occurs, how do the masses of the reactants compare to the products? In this experiment you will use the same number of MOLES of the reactants, lead II nitrate and potassium chromate and they will be allowed to react with each other. You will be comparing the mass of the two reactants with the mass of the products. You will also compare the moles of reactants to the moles of products.
You will use the electronic balance again along with some new techniques and devices such as, filtering, decanting, washing and drying. Learn how to execute or use them since you will be expected to know them in future labs. If you have any questions about them ask your instructor to explain them again.
Make sure your data table is neat, orderly and complete. Your data table is going to consist of a list of anything you measured in lab. Remember to use uncertainty and units.
PRELAB ASSIGNMENT
Determine the mass in grams of 0.00500 moles of potassium chromate and 0.00500 moles of lead II nitrate. You must show these calculations to your instructor as well as placing them in your lab write up.
PROCEDURES
1. Find the mass of a clean, dry beaker (250 ml or >).
2. Place a piece of weighing paper on the balance and TARE it. (the balance should read zero with the paper)
3. Carefully tip and tap out enough crystals of potassium chromate to equal the mass you determined in the prelab assignment. Record the mass in your data table.
4. Transfer the potassium chromate to the massed beaker.
5. Add about 25 ml of distilled water to dissolve the potassium chromate. If you wish you may slowly and carefully warm the solution to aid in the dissolving.
6. Repeat steps 1-5 using a different beaker and lead II nitrate. This beaker should be at least 400ml.
7. Add about 1 ml of lead II nitrate solution to the potassium chromate solution and stir. Be careful not to splash. What do you observe?
8. While stirring, slowly add the rest of the lead II nitrate solution A FEW MILLILITERS AT A TIME. Rinse the remaining lead II nitrate solution with a few ml of distilled water and pour this into the reacting beaker.
9. Heat the solution in the reacting beaker for about five minutes. Do not let it boil vigorously!! You should see a spectacular show inside the solution!!
10. Allow the precipitate to settle.
11. Mass a piece of filter paper and record in the data table.
12. Fold the filter paper, place inside a funnel and set up the apparatus as your instructor demonstrated. If you moisten the filter paper with some distilled water it will adhere to the sides of the funnel.
13. Place the rinsed out beaker that contained the lead nitrate solution under the funnel. This will catch the filtrate. DECANT the clear liquid from the reaction vessel into the filter paper. Retain as much of the precipitate (ppt) as possible in the reaction vessel.
14. Wash the ppt in the beaker with about 20 ml of distilled water and warm the mixture again. Warm the solution for about 3 minutes. Let the ppt settle and decant the liquid again into the funnel. Leave as much of the ppt as possible in the beaker.
15. Wash the ppt again with 15 ml of distilled water and decant into the funnel.
16. When the filtrate has stopped dripping from the funnel, carefully remove the filter paper and place it inside the beaker with the ppt. Write your name on the side of the beaker with the pen your instructor provided and place the beaker in the drying oven. You will weigh the dry beaker, filter paper and ppt tomorrow.
17. The filtrate that has been collected should be clear and colorless. But is it just water? You can find out by setting up a ring stand, ring and Bunsen burner and evaporating the liquid. Do this and record your observations.
18. Weigh the beaker after it has cooled enough to hold in your hand. Record this mass in the data table.
NEXT DAY
19. Now obtain your beaker, filter paper and ppt from the drying oven and weigh it dry. Record in the data table.
Substance / Mass / MolesLead II nitrate / .00500
Potassium chromate / .00500
Lead II chromate
Potassium nitrate
CALCULATIONS
1. Determine the mass of each product. The yellow ppt is lead II chromate and the dry residue from the filtrate is potassium nitrate.
2. Set up a table like below and show the necessary calculations to fill it in.
3. Compare the sum of the masses of the products to the sum of the masses of the reactants by setting up a ratio:
mass of products (lead II chromate + potassium nitrate)
mass of reactants (lead II nitrate + potassium chromate)
Your answer should be in decimal form and don’t forget sig. figs.!!
4. What does the value of the ratio from question 3 mean?
5. Write an equation for the reaction by placing the formulas for the two reactants on the left side of an arrow and the formulas for the two products on the right side of the arrow.
6. Below each substance write the number of moles used or formed. This is found in the table from question 2.
7. The numbers you have written below the substances in question 6 are really not very easy to deal with. A trick used by chemists is to change them into whole numbers. This is accomplished by dividing all values by the smallest of them. This now makes the smallest value 1. To ensure continuity with other students we will assume the smallest value you obtained was 0.00500 moles, therefore we will all divide by this value. Do this with your values and rewrite the equation using whole numbers.
8. When a quantity does not change during a chemical reaction we say it is conserved. Use your results and data to answer the following questions:
A. Is mass conserved in this reaction?
B. Are atoms conserved in this reaction?
C. Are moles conserved in this reaction?