Gravimetric analysis- sample calculations  

A 3.46 g sample of limestone(CaCO3 ) was dissolved in 0.1M (HCl) solution according to the following equation.

CaCO3 (s) + 2HCl (aq) => Ca2+(aq) + 2Cl -(aq) + CO2(g) + H2O (l)

Excess 0.1M (NH4)2C2O4(aq), was added to the resulting solution to precipitate the calcium ions as calcium oxalate, CaC2O4(s).

The precipitate was filtered, dried and weighed at 2.03 g.

Determine the percentage by mass of calcium in the limestone sample.

Step 1 Write the balanced ionic equation for the precipitation reaction: Solution

2) Visualise what is happening and complete the flow chart on the right with the steps below.
a) Add HCl
b) Crush the limestone
c) Weigh the sample of limestone
d) Add excess 0.1M ammonium oxalate
e) Dry the precipitate
f) Wash the precipitate
g) Filter the precipitate
h) Weigh the precipitate.

Step 2 Calculate the mole of precipitate formed
Step 3 Find the mole of Ca2+ ions in step 1) the ratio of Ca2+ reacting and CaC2O4 produced is 1:1 so the mol of Ca2+ is 0.0158.  
Step 4 Find the mass of Ca2+.
Step 5 Calculate the percentage, by mass, of calcium in the original limestone sample.
1) What are the properties of an ideal precipitate that can be used in gravimetric analysis.
2) The precipitate trapped in the filter paper is usually washed thoroughly. In this case it is to
3) A precipitate that is not thoroughly washed tends to give results that are
4) When washing the precipitate, a small sample of the filtrate is periodically tested with silver nitrate solution. Why?

5) In one gravimetric analysis a precipitate of silver chloride is formed by adding silver nitrate to a sample containing chloride ions, as shown on the right.

a) Which ion is the contaminant and where did it come from?

b) Why can the contaminant be washed off the precipitate, without washing some of the precipitate ions away as well?

c) If the precipitate was slightly soluble in water, during the washing, we would expect to

Continue with secondhand data