chemistry
posted by anonymousokm .
A sample being given as a laboratory unknown is a mixture of calcium carbonate and magnesium carbonate. Both substances decompose to form calcium oxide and magnesium oxide, respectively, when heated at a high temperature for a prolonged period. An unknown sample weighing 5.424 grams was decomposed by heating for several hours at 950 °C. The oxide residue which remained weighed 2.791 grams. From this laboratory data calculate the percent, by weight, of calcium carbonate in the unknown.

I do these with two unknowns but they can be worked with just one (but it doesn't help the math).
Let x = mass CaCO3
and y = mass MgCO3

x + y = 5.424
[x(molar mass CaO/molar mass CaCO3)] + [y(molar mass MgO/molar mass MgCO3)] = 2.791
Two equations and two unknowns. Solve for x and y (although the question doesn't want to know anything about y).
Then %CaCO3 = (mass CaCO3/mass sample)*100 = ?
Post your work if you get stuck. 
thx a lot for all the help ..i got the answer 44.4 % :D