- When a chemical reaction occurs, how do the masses of reactants and products compare? In this experiment, you will use the same number of moles of two reactants, lead nitrate, Pb(NO3)2 and potassium chromate, K2CrO4 , and allow them to react with each other. You will be making a comparison between the masses of reactants and products and the number of moles of reactants and products.
- You will learn the techniques of decanting, filtering, washing, and drying. Carry them out well, so you will be able to perform these laboratory procedures in future labs.
- Record your data carefully and neatly. Take special care to show the units used in your measurements.
- Before you come to the lab to do an experiment, you should plan what you are going to do. This preparation will free your mind from mechanical details and allow you to concentrate on making the required observations in the allotted time.
PRECAUTIONS:
WEAR SAFETY GOGGLES THROUGHOUT THIS EXPERIMENT!
Avoid allowing chromates to contact the skin, since they sometimes cause skin rashes.
Avoid contact with nitrates as they react with the skin.
MATERIALS: 250-ml beaker Lead nitrate Pb(NO3)2 250-ml Erlenmeyer flask Potassium chromate K2CrO4 2 small weighing dishes Hot plate Funnel Ring stand 1 circle-filter paper Small ring clamp Balance that masses to 0.01g Rinse bottle with distilled water *Desiccating oven **Crucible tongs
*The desiccating oven only has to get just below 100 degrees C. An incubator turned all the way up is perfect.**Never use beaker tongs with a flask. The flask is tapered, and you will end up with a broken flask and hot chemicals on you. Crucible tongs securely pick up a flask by the neck, and the lip prevents it from slipping.
PROCEDURE:
PRE-LAB
1. Determine the mass in grams of one mole of potassium chromate, K2CrO4 , one mole of lead nitrate, Pb(NO3)2 , one mole of lead chromate, PbCrO4 , and one mole of potassium nitrate, KNO3 . Round your calculations to 3 significant figures and record in the DATA TABLE.
2. Determine the mass in grams of 0.00500 mole of K2CrO4 and 0.00500 mole of Pb(NO3)2 . Record your answers on the Table in the CALCULATION SECTION.
IN THE LAB
DAY 1
3. Using a pencil, mark a clean, dry 250-ml beaker with your name, and find its mass to the nearest 0.01 g. Record on the DATA TABLE.
4. With the proper procedure for the balance you are using, carefully weigh out 0.00500 mole of K2CrO4 in a weighing dish ( this is the mass you calculated in step 2 and recorded in the table of the calculation section).
5. Transfer the K2CrO4 to the 250-ml beaker. Rinse the weighing dish into the beaker, and add about 25 ml of distilled water. Warm the solution, on the hot plate, to increase the rate of dissolving. Do not let it boil!
6. Mark a clean, dry 250-ml Erlenmeyer flask with your name, and find its mass to the nearest 0.01 g . Record on the DATA TABLE.
7. Using the procedure in step 4, and a dry weighing dish, obtain the correct mass of 0.00500 mole of Pb(NO3)2 (again, this is the mass you calculated in step 2 and recorded in the table of the calculation section).
8. Transfer the Pb(NO3)2 to the Erlenmeyer flask, rinse the weighing dish into the flask, and add about 25 ml of distilled water. Warm the solution to increase the rate of dissolving. Do not let it boil!
9. Add about 1 ml of the lead nitrate solution from the flask to the potassium chromate solution in the beaker and gently swirl the beaker about 1 minute. Avoid splashing by keeping the beaker on the table. This lets you control the process.
10. While continuing to gently swirl the beaker, add the rest of the lead nitrate, a few milliliters at a time. Rinse the last traces of lead nitrate from the flask into the beaker with some distilled water, using a wash bottle. Use as little water as is practical. Several rinses of small volume are more effective than a single one of larger volume. Do at least four rinses to remove all the lead nitrate to the beaker. Heat the solution for about five minutes, but do not let it boil vigorously. Allow the Precipitate to settle. Place the beaker and flask in a secure location for 24 hours.
DAY 2
11. Determine the mass of a piece of filter paper to the nearest 0.01 g . Record the mass on the DATA TABLE. Set up the filtering funnel using the ring stand and the small ring clamp to hold it over your empty E-flask. Fold the filter paper in half, then in fourths. Open the filter paper so there is one thickness on one side, and three thicknesses on the other side. Hold the filter paper securely in the funnel, and wet it with the rinse bottle to stick it in place.
12. Put the flask under the funnel. Decant the clear liquid from the beaker into the funnel. Retain as much of the precipitate as possible in the beaker. Decanting means to pour off the liquid but retain the solid precipitate. The less precipitate that gets in the filter paper, the quicker this will go.
13. Wash the precipitate in the beaker by adding about 20 ml of distilled water and swirling the beaker as before. Warm the mixture again, just until the precipitate begins to look grainy and a little darker orange in color. 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 beaker with another 15 ml of distilled water. Warm and let settle as before. Again decant the wash water into the filter paper in the funnel.
14. Wash the sides of the beaker with 10 ml of distilled water, warm, let settle, and decant this also into the filter paper.
15. Using the rinse bottle, rinse the filter paper, from the top edge down, with 4 small rinses. When the filtering is complete, remove the flask and place it on the hot plate. This is to evaporate the filtrate and obtain the dry crystals. Do not let it boil to hard.
16. Remove the filter paper from the funnel, being careful not to loose any precipitate that is in it, and place it loosely into the beaker. Place the beaker in the desiccating oven to dry the precipitate over night.
17. Watch the flask on the hot plate closely. As it boils dry, white crystals will form on the bottom. Promptly remove the flask from the hot plate, with crucible tongs, or the chemical will be damaged (there will be a small amount of water condensed in the neck of the flask). Be careful it is hot! If another day is needed to get the filtrate dried, that is ok. After removing the flask from the hot plate, place it in the desiccating oven with the beaker, to dry over night.
DAY 3
18. Remove the beaker and flask from the desiccating oven and set them on the counter to cool. When they are room temperature, find the total mass of beaker, filter paper, and precipitate to 0.01 g. Record the mass on the DATA TABLE. Then find total mass of flask and residue to 0.01 g. Record the mass on the DATA TABLE.
DATA TABLE:
Mass of one mole of potassium chromate, K2CrO4 = __________ g
Mass of one mole of lead nitrate, Pb(NO3)2 = __________ g
Mass of one mole of lead chromate, PbCrO4 = __________ g
Mass of one mole of potassium nitrate, KNO3 = __________ g
Mass of 250-ml beaker. = __________ g
Mass of 250-ml Erlenmeyer flask. = __________ g
Mass of the filter paper. = __________ g
Total mass of beaker, filter paper, and precipitate. = __________ g
Total mass of flask and residue. = __________ gCALCULATIONS:
CALCULATION TABLE Chemical Mass in grams Number of Moles Pb(NO3)2
0.00500
K2CrO4
PbCrO4
KNO3
19. Using the masses from your DATA TABLE, determine the mass of each product. You simply subtract away the unwanted masses of beaker, filter paper, and flask, from the appropriate data. The yellow precipitate is lead chromate, PbCrO4 .The dry residue from the filtrate is potassium nitrate, KNO3 . Record your answers on the CALCULATION TABLE.20. Calculate the number of moles of each product and record your answers on the CALCULATION TABLE.
21. Compare the sum of the masses of the reactants to the sum of the masses of the products by finding the ratio. The formula is as follows:
mass of products [PbCrO4 + KNO3] mass of reactants [Pb(NO3)2 + K2CrO4]
- Express your answer as a decimal and use the correct number of significant figures. Remember, this number represents the top of the division, or the mass of the products. The mass of the reactants was on the bottom of the division, so it is represented by 1. There are no units because the grams canceled.
Mass of reactants : Mass of products ____1____
:
_________
22. For the formula below, write the number of moles used or formed. This is the data you have in your CALCULATION TABLE.Pb(NO3)2 + K2CrO4 ---> PbCrO4 + KNO3
________
________
________
________mole mole mole mole
23. Use the information from question 22 to balance the following equation ( you will find it helpful to divide each of the mole quantities obtained in question 22 by 0.00500 in order to do this ). Round the answers to the nearest whole number.1 mole of Pb(NO3)2 (in solution) + 1 mole of K2CrO4 (in solution) ----->
_____ mole of PbCrO4 (solid) + ______ mole of KNO3 (solid)
24. When a quantity does not change during a chemical reaction, we say that it is conserved. Use your answers to calculations 21 and 23 to answer the following:
Keeping in mind, experimental error.
(a) Is mass conserved in this chemical reaction? _________
(b) Are atoms conserved in this chemical reaction? _______
(c) Are moles conserved in this chemical reaction? _______
Back to Chemistry Table of Contents
Back to Lab Dad's Laboratory
- You will learn the techniques of decanting, filtering, washing, and drying. Carry them out well, so you will be able to perform these laboratory procedures in future labs.
MASS RELATIONSHIPS
ACCOMPANYING CHEMICAL CHANGES
INTRODUCTION: