If 40.0 grams of H3PO4 reacts with 60.0 grams of MgCO3 according to this reaction, 2H3PO4 + 3MgCO3 —> Mg3 (PO4)2 + 3CO2 + 3H2O (already balanced) then calculate the volume of carbon dioxide gas produced at STP.

Step 1 - use the molar masses to determine the number of moles of H3PO4 and MgCO3 that you have.
Step 2 - Determine which you will run out of first in the above reaction.
Step 3 - Figure out how many moles of carbon dioxide will be produced.
Step 4 - Use the ideal gas law to figure out what the volume of that amount of CO2 will be.

Run through these steps and post your answer for us to review.