Name:
Date:
Chemistry Experiment 7
Mass Relationships Accompanying Chemical Changes
Purpose:
1. To determine if mass is conserved in a chemical reaction.
2. To determine if moles are conserved in a chemical reaction.
Materials:
1. 0.00500 moles of Na2CrO4
2. 100 mL beaker
3. Filter Paper
4.
5.
6.
0.00500 moles of Pb(NO3)2
Ring Stand
Wash Bottle
7.
8.
9.
150 mL beaker
Filtering Funnel
Distilled Water
Safety:
1. Do not over heat nitrates as they are explosive. You also do not want to lose your eyesight if your
neighbour's beaker explodes and glass gets in your eye. Always wear safety glasses even when you
are done your experiment.
2. Chromates are very hazardous. Report any spills immediately, especially if it is on one of the
balances.
Procedures:
1. Mass a clean, dry and labelled 150 mL beaker to the nearest 0.01 grams. Record this mass.
2. With the beaker on the analytical balance press the reset. (The balance should read 0.000 grams). Add
0.00500 moles of Na2CrO4 to the beaker. (0.00500 moles of Na2CrO4 = ______ ± 0.001 grams)
3. Mass a clean dry labelled 100 mL beaker to the nearest 0.001 grams. Record this mass.
4. With the beaker on the analytical balance press the reset. (The balance should read 0.000 grams.) Add
0.00500 moles of Pb(NO3)2 to the beaker. (0.00500 moles of Pb(NO3)2 = _______ ± 0.001 grams.
5. Dissolve each of the solids in about 25 mL of water. Warms the solutions if desired to increase the rate
of dissolving.
6. Add 1 mL of the Na2CrO4 solution to the Pb(NO3)2 solution and stir for one minute. Avoid splashing.
7. While continuing to stir the solution, add the rest of the Na2CrO4 solution, a few milliliters at a time.
Rinse the last traces of Na2CrO4 from the 150 mL beaker with some distilled water. Use as little water
as is practical. Several rinses of small volume are more effective than a single larger volume. Heat the
solution for about 5 minutes, until clear, do not let boil vigorously. Allow the precipitate to settle.
8. Determine the mass of a piece of filter paper to the nearest 0.001 grams. Fold the filter paper as was
demonstrated and fit it into a funnel and moisten it with some distilled water. Set up the funnel for
filter as was demonstrated.
9. Put the 150 mL beaker under the funnel. Decant the clear liquid from the 100 mL beaker into the
funnel. Retain as much of the precipitate as possible.
10. Wash the precipitate in the beaker by adding about 20 mL of distilled water and warming the mixture
again. Let the precipitate settle and decant the liquid into the funnel leaving as much of the precipitate
as possible in the beaker. Wash the precipitate in the 100 mL beaker with another 15 mL of distilled
water.
11. Again decant the wash water into the filter paper in the funnel. Wash the sides of the beaker with 10
mL of distilled water and pout this into the filter paper. It is not necessary to attempt to transfer of the
precipitate to the filter paper, since you will later place the filter paper and its contents in the beaker to
be dried.
12. When filtering is done place the unfolded filter paper into the 100 mL beaker and place it into the
drying oven overnight.
13. Place the filtrate in the 150 mL beaker into the drying oven with the 100 mL beaker.
14. Next day determine the mass of the 100 mL beaker, filter paper and precipitate. Also determine the
mass of the 150 mL and the NaNO3.
Name:
Date:
Data:
Mass of labelled 150 mL beaker
Mass of labelled 100 mL beaker
Mass of filter paper
Mass of Na2CrO4
Mass of Pb(NO3)2
__________________ ± 0.01 grams
__________________ ± 0.001 grams
__________________ ± 0.001 grams
__________________ ± 0.001 grams
__________________ ± 0.001 grams
Mass of 150 mL beaker & NaNO3
Mass of 100 mL beaker, filter paper & PbCrO4
__________________ ± 0.01 grams
__________________ ± 0.001 grams
Questions:
1.
Determine the mass of each product.
2.
3.
Copy the following table and make the necessary calculations to fill in the entries.
Reactant or Product
(NaNO3 & PbCrO4)
Mass
Number of Moles
Pb(NO3)2
0.00500
Na2CrO4
0.00500
PbCrO4
NaNO3
4.
Compare the sum of the masses of the reactants to the sum of the masses of the products by finding
the ratio:
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠
Express your answer as a decimal and use the correct number of significant figures.
5.
What is the significance of the ratio calculated in question 3?
6.
What should the ratio be calculated in question 3?
7.
Calculate your percent error using your ratio as the experimental and the answer to question 5 as the
accepted.
8.
Complete the following chemical equation using your data.
0.00500 moles Pb(NO3)2 + 0.00500 moles Na2CrO4 → _____ moles PbCrO4 + ____ moles NaNO3
9.
Use the information from question 7 to balance the following equation using small whole numbers
______ Pb(NO3)2 + ______ NaCrO4 → ______ PbCrO4 + ______ NaNO3
10. When a quantity does not change during a chemical reaction, we say that it is conserved. Use your
answers to questions 3 and 8 to answer the following:
a) Is mass conserved in this chemical reaction?
b) Are atoms conserved in this chemical reaction?
c) Are moles conserved in this chemical reaction?
Sources of Error:
Conclusion: