Question

How do you do mass to mass stoichiometry problems?

Answer 1

I'm assuming you mean reacting masses - for example:
"Calculate the amount of `CO_2` formed when 58g of `CaCO_3` decomposes."

So first you need to write a balanced equation:
`CaCO_3 -> CaO + CO_2`

Then you need to divide the mass you are given by its molar mass, to find the number of moles that react, so..
Molar mass `CaCO_3` = 40 + 12 + (3 x 16) = 100
Moles of `CaCO_3` reacting = 58g / 100 = 0.58

Then you need to multiply the number of moles of `CaCO_3` by the molar mass of the thing you have to find (`CO_2`) so...
Molar mass `CO_2` = 12 + (1 x 16) = 44
Mass `CO_2` = 0.58 x 44 = 25.5g.

If you have different stoichiometric ratios, it is simplest to multiply the molar mass by the coefficient first, so if you had a question, "Calculate the mass of `Fe_2O_3` formed when 10.0g of Oxygen reacts with excess iron"...

First write that balanced equation:
`4Fe + 3 O_2 -> 2 Fe_2O_3`

Then divide the moles of `O_2` given by the molar mass x 3, so...
Molar mass `O_2` = 2 x 16 = 32
Moles = 10 / (32 x 3) = 0.10417

Then multiply this number by the mass of `2 Fe_2O_3`:
Molar mass `Fe_2O_3` = (2 x 56) + (3 x 16) = 160
Mass `Fe_2O_3` formed = (160 x 2) x 0.10417 = 33.3g