Question

I have 2 samples of solid chalk (aka calcium carbonate). Sample A has a total mass of 4.12 g and Sample B has a total mass of 19.37 g. What is the difference between the samples?

A) Sample B has more calcium carbonate molecules
B) Sample B has a larger ratio of carbon, oxygen, and calcium atoms
C) Sample B has more calcium ion than carbonate ions
D) Sample B must have some impurity

Answer 1

A) Sample B has more calcium carbonate molecules

Step-by-step explanation:

M = Molar mass of calcium carbonate = 100.0869 g/mol

`N_A` = Avogadro's number =
`6.022* 10^(23)\ \text{mol}^(-1)`

For the 4.12 g sample

Moles of a substance is given by

`n=(m)/(M)\\\Rightarrow n=(4.12)/(100.0869)\\\Rightarrow n=0.0411\ \text{mol}`

Number of molecules is given by

`nN_A=0.0411* 6.022* 10^(23)=2.48* 10^(22)\ \text{molecules}`

For the 19.37 g sample

`n=(19.37)/(100.0869)\\\Rightarrow n=0.193\ \text{mol}`

Number of molecules is given by

`nN_A=0.193* 6.022* 10^(23)=1.16* 10^(23)\ \text{molecules}`

`1.16* 10^(23)\ \text{molecules}>2.48* 10^(22)\ \text{molecules}`

So, sample B has more calcium carbonate molecules.

The ratio of the elements of carbon, oxygen, calcium atoms, ions, has to be same in both the samples otherwise the samples cannot be considered as calcium carbonate. Same is applicable for impurities. If there are impurites then the sample cannot be considered as calcium carbonate.