Lab #4 – Gravimetric Analysis
Name
Background
You will be given an unknown salt that is either a metal carbonate or a metal bicarbonate; your job will be to
determine its chemical formula. You will do this by reacting the salt with hydrochloric acid in the classic acidcarbonate reaction, made famous by the reaction between vinegar (acetic acid) and baking soda (sodium
bicarbonate), the fizzy reaction so popular in grade school science projects.
Compare the following balanced equations:
HCl(aq) + CaCO3(s)  CaCl2(aq) + CO2(g) + H2O(l)
note the Cl- : CO3-2 ratio is 2:1
HCl(aq) + Ca(HCO3)2(s)  CaCl2(aq) + CO2(g) + H2O(l)
note the Cl- : HCO3-1 ratio is 1:1
Notice the three products in both reactions are identical. The CO2 gas will escape and you will carefully boil
away the water, leaving behind the chloride salt produced. By weighing the chloride salt produced you can
calculate the moles of chloride in the product. We will tell you the identity and mass percent of metal in your
unknown, something that can be determined using a separate lab procedure; this will allow you to determine the
mass (and moles) of metal in your unknown, and therefore moles of carbonate/bicarbonate. By finding the ratio
of chloride:carbonate/bicarbonate, you can decide whether your unknown was a carbonate salt or a bicarbonate
salt. Using that information and your gravimetric (weight) data, you will be able to calculate the formula of
your unknown compound.
Procedure
Obtain a sample of an unknown carbonate or bicarbonate salt. Your instructor will provide you with the
identity and mass percent of the metal in the unknown salt; record both pieces of information and the unknown
number of your sample.
Clean your crucible and lid thoroughly with detergent, tap water, and a brush. Rinse thoroughly with tap water,
followed by a distilled water rinse. [Note: Nitric acid will be available for cleaning very dirty crucibles.] Dry
the crucible with paper towel. Complete the cleaning process by heating your crucible/lid with a Bunsen
burner; heat gently with the lid slightly ajar for a couple of minutes, then heat aggressively for 2 more minutes.
Once the crucible is clean, do not touch it with your fingers; use paper towel to grip it. Allow the crucible/lid to
cool to room temperature, then weigh the crucible/lid on the analytical balance and record the mass. Using the
top loading balancing, weigh approximately 0.3 g of your unknown on weighing paper. Transfer the sample to
your crucible and reweigh on the analytical balance. Record the mass of the crucible, lid, and sample.
Set up the ring stand with ring and clay triangle. Place the crucible with sample in the clay triangle. Add
approximately 10-20 drops of 6 M HCl drop-wise to the sample. Keep adding until no more fizzing occurs. Be
careful not to foam over or splatter. Tilt the crucible around so that the HCl can react with any sample stuck to
the inside walls of the crucible. Heat the crucible gently to complete the reaction. Add a few more drops of
HCl only if the solid is not completely dissolved.
With the lid slightly ajar, heat the crucible gently to boil off the water, moving the burner in and out by hand to
control the rate of heating. Be careful to avoid spattering. Be careful not to inhale the excess HCl fumes that
will be released at this point. Once the solid is dry, heat aggressively for an additional 10 minutes; the crucible
should glow red hot. Allow the sample to cool for at least 10 minutes, then weigh and record the mass.
Prelab summary
Procedural Tips: Add HCl slowly to avoid foaming over and splattering.
Boil off water slowly to avoid spattering.
Do NOT place a hot crucible directly on a cold lab bench. Use the ceramic pad from the community drawer.
Do NOT weigh a hot object on an electronic balance.
You have two types of tongs and forceps at your disposal for handling labware. Choose intelligently.
Safety: Use caution with 6 M HCl. Rinse hands with tap water after each use. Contact instructor immediately
concerning spills. Avoid HCl fumes, especially when heating.
Do NOT use excess HCl as any excess will be blown off as nasty HCl fumes later.
Hot glass and ceramic ware look exactly like cold glass and ceramic ware.
Treat the $2,000 analytical balance like the extremely delicate instrument it is.
Disposal: All excess reagents can be rinsed down the sink with plenty of tap water.
Data and Calculations
UK #
Metal in UK
Mass % metal
Mass of crucible/lid
g
Mass of crucible/lid and sample
g
Mass of sample
g
Mass of crucible/lid and chloride product
g
Mass of chloride product
g
Mass of metal in UK and chloride product
g
Moles of metal in UK and chloride product
mol
Mass of Cl- in chloride product
g
Mole of Cl-
mol
Mole ratio, metal:chloride in product
Nearest integral mole ratio
:
:
Mass of carbonate/bicarbonate in UK
Moles, if carbonate
%
Formula of chloride product
g
mol
Moles, if bicarbonate
Mole ratio of chloride in product:carbonate/bicarbonate in UK
mol
:
The UK is a carbonate bicarbonate (circle one)
Mole ratio, metal:carbonate/bicarbonate in UK
:
Formula of UK
Prelab Questions (Show ALL work)
A student using this procedure collects the following data: crucible/lid = 21.462 g
crucible/lid + UK = 21.787 g
crucible/lid + chloride product = 21.830 g
He is told his unknown carbonate/bicarbonate is 28.8% magnesium
1. Find the mass and moles of magnesium in the UK and the product.
g
mol
2. Find the mass of carbonate/bicarb UK and the mass of chloride product
g UK
g product
3. Find the mass and moles of carbonate/bicarbonate in the UK
g
mol
4. Find the mass and moles of chloride in the product
g
mol
5. Calculate the ratio of Cl- in product:carb/bicarb in UK
6. Is the unknown a carbonate or a bicarbonate? Explain.
:
carbonate OR bicarbonate
(circle one)
7. What is the formula of the UK?
8. What is the formula of the product?
9. If some of the chloride product is lost due to spattering, how will the determination of the UK’s formula be
affected? Be specific as you discuss the resulting changes in calculation.