For the balanced chemical reaction C3H8 + 5O2 → 3CO2 + 4H2O how many grams of C3H8 are needed to make 10.7 g of H2O? Express your answer to three significant figures.

Question

Looking for the steps to solve this equation.

I got 470.8 grams of C3H8

Answer 1

What's the first thing you do? Convert to moles. But you only need the molar values of the relevant compounds C3H8 and H2O. To do this, consult the periodic table for each element. For C3H8 -> ((3*12)+(1*8)) = 44 g/mol and H2O is ((2*1)+(16)) = 18 g/mol. To get to moles, divide the gram amounts by the molar weights. We have water H2O being 10.7 g / (18 g/mol), giving us 0.5944 mol H2O. Next look at the stoichiometric ratio of water to propane (C3H8), which is 4:1 based on the coefficients. That means that for every one mole of propane used, four moles of water are created. Therefore, if we need to make 0.5944 moles of H2O, we will need a quarter of that for propane, or 0.1486 moles C3H8. Since we need to convert back to grams of C3H8, we simply multiply the moles by the molar weight, or 0.1486 mol*(44 g/mol) -> 6.54 g C3H8.

In essence -> convert everything to moles -> look at stoichiometric coefficients to see how much is needed per how much is used -> convert back to grams.