 Let's go a step further with our stoichiometry. In the last example we did a limiting reactant problem but we used only moles. What if we bring in masses? Here's the equation for the combustion of acetylene, the fuel that burns in acetylene welding torches. Getting the oxygen-fuel mixture right in a welding torch is really important in order to get the hottest possible flame. So, say we're going to combust 1.3 grams of acetylene with 1.5 grams of oxygen. Which is the limiting reactant and then calculate what mass of carbon dioxide would be produced? This is a multi-part calculation. The first thing to do is to sort out what steps are going to be needed. You're given masses of reactants and you're asked to calculate a mass of product so you're going to have to use the land of the mole. But I'm going to modify it a bit. Since we're going from mass of reactant to mass of product I'll take off the particle and volume calculations because we don't need those. Now we have an additional complication. We don't know which of the two reactants we should use to determine the moles of product because we don't yet know which one is limiting. So I'm going to shift this around again so that I'm now showing the mass-mol conversion for both of my reactants here on the left. Okay, so the steps that we need to go through are first of all we convert the masses of each reactant into moles. Once you know this you can work out which reactant is limiting and you can then ignore the excess reactant and use the limiting reactant for the rest of the calculation. You take the moles of the limiting reactant and you apply the mole ratio to calculate the moles of product formed. And finally you do a mass-mol conversion to convert the moles of product to a mass of product. So let's do that. Step one, we'll convert both the reactant masses to moles. I'll work out their molar masses and then divide by that to get moles for each of them. Step two is to work out which is the limiting reactant. Now the mole ratio here is two acetylins to five oxygens. So I put in the moles of acetylene present, remember I could do that with either reactant and I use that mole ratio to work out how much oxygen would be needed to exactly react with it. And I find that 0.1248 moles of oxygen is needed to completely react with the amount of acetylene that I have. Now I compare that value with the amount of oxygen that is present which is 0.04688 moles. So what I have present is less than what I need. So that means the oxygen will run out first and it is my limiting reactant. And I now have what I need to do the rest of the calculation. So step three is to calculate the amount of carbon dioxide formed. I know my limiting reactant, that's oxygen, and that's going to determine how much of the CO2 is formed. So I need to use the mole ratio to work out the moles of product and then do a mole mass conversion to find the mass of product. The amount of oxygen I have is 0.04688 moles. Be very careful at this point to put in the amount that you actually have present, not the hypothetical amount that you would need to react with the excess reactant. The mole ratio is 5 oxygens to 4 carbon dioxide. So I take the amount of oxygen and I divide by 5 and multiply by 4 to find out how many moles of carbon dioxide are produced. And this gives me 0.03750 moles of carbon dioxide. The final task is to convert this into a mass. So I need the molar mass of CO2. I pull out the periodic table and figure out that that's 44.01 grams per mole. And I multiply the moles of CO2 by the molar mass to get the mass of CO2, which is 1.650 grams. And since my original masses were to 2 sig figs, I round that down to 1.7 grams. So there you are. That's quite a long calculation, but when it's broken down into parts, each part is quite simple. The most difficult bit of stoichiometry is working out your strategy. What calculations do I need to string together to solve this problem? So remember these tips. Write down what you know and what you're aiming for from the information in the question. Draw up a calculation strategy by breaking the problem down into chunks. Use the land of the mole to help with this. Annotate your calculations with units and formulae to keep track of where you are and what everything means. And lastly, practice.