AP Chemistry Help, an actual AP problem?

Question

I am having a lot of trouble with this problem. Could someone help me find an answer and explain why it is

The question is:

A sample of dolomitic limestone containing only CaCO3 and MgCO3 was analyzed.

a) When a 0.2800 gram sample of this limestone was decomposed by heating, 75.0 milliliters of CO2 at 750 mm Hg and 20 degrees C were evolved. How many grams of CO@ were produced

b) Write equations for the decomposition of BOTH carbonates described above.

c) It was also determined that the initial sample contained 0.0448 grams of calcium. What percent of the limestone by mass was CaCO3

d) How many grams of the magnesium-containing product were present in the sample in (a) after it had been heated?

Thanks in advance!!!

Answer 1

CaCO3 --> CaO + CO2

MgCO3 --> MgO + CO2

n = PV/RT = 0.9868 atm x 0.075 L / (0.082057 x 293) = 0.003078 mol

0.003078 mol x 44.01 g/mol = 0.1355 g CO2

a) 0.1355 g CO2

0.0448 g Ca / 40.08 g/mol = 0.001118 mol Ca

0.001118 mol CaCO3 x 100.1 g/mol = 0.1119 g CaCO3

c) 0.1119 g CaCO3

0.2800 - 0.1119 = 0.1681 g MgCO3

d) 0.1681 g MgCO3

Answer 2

It's just simple stoichiometry.

A) Use PV=nRT to determine moles of CO2, use molecular weight to determine mass CO2.  Watch your units.

B) CaCO3 is the reactant.  CO2 is one product.  Atoms have to balance, right?  So "subtract" CO2 from CaCO3, what Ca-containing byproduct will you be left with?  MgCO3 is similar.

C) Mass Ca gives you moles Ca.  Moles Ca gives you moles CaCO3.  Moles CaCO3 gives you mass CaCO3.  Mass CaCO3 and total mass of sample gives you %mass CaCO3.

D) Mass CaCO3 and total mass of sample gives you mass and moles MgCO3.  Moles MgCO3 gives you moles Mg product (which you figured out in B).  Moles Mg product gives you mass Mg product.

Edit: note to Ronald.

Don't do his homework for him, man, he won't learn anything that way.  As well, you're only allowed three significant figures in your answers (assuming we take 750mmHg as +/- 1).