# Gravimetric Calculations

Gravimetric analysis is a technique through which the amount of the ion being analyzed can be determined through the measurement of mass. Gravimetric analysis depend on comparing the masses of two compounds containing the ion to be analyzed.

We’re going to work through a standard question to really understand how this works. What this question is really saying is that if you know the mass of something (an ion) in a pure substance, you can figure out the mass percent of that same ion in an impure sample if you know the mass of the impure sample. This only works for ions that can be completely precipitated and easily filtered, like the limestone in the video, or by using silver chloride to determine the % Cl-.

Here’s an example:

A 3.46 g sample of limestone(CaCO3 ) was dissolved in 0.1M (HCl) solution like this:

4 Responses to “Gravimetric Calculations”
1. Aurora says:

Yes that’s right. Just to be clear:

When you add up all the different molar masses

Ca = 40
C = 12
O3 = 16 x 3 = 48

You get 100. And 40/100 = 40%.

2. ebrice says:

My mistake, I just watched the video and missed the part about it being an IMPURE sample. If you have PURE CaCO3, would the percent by weight of Calcium be 40%?

3. Aurora says:

Hi there! Tell me more about what you’re asking – I am not quite understanding your question.

The principle behind this technique is that the mass of the ion in a pure compound can be found and used to find the mass % of the same ion of an unpure compound if you know how much of the unpure substance you have. You must completely precipitate it and filter it in order for this to work.

4. ebrice says:

Hi

Why isn’t the percent of calcium in EVERY sample of CaCO3 by weight 40%? (For every molecule there is 1 calcium atom, 1 carbon atom and 3 oxygen atoms).