A 50.6 g sample of Mg(OH)2 is reacted with 45.0 g of HCl according to the reaction:
Mg(OH)2 + 2 HCl --> MgCl2 + 2 H2O
What is the theoretical yield of MgCl2?
Is this a limiting reagent problem? One way to find out is to write down what is known about any component of the reaction below that component:
Notice how quantities of both reactants are known. Which one will be used up first? You can't tell, nor should you jump to any conclusions. Just because it looks like there is less Mg(OH)2 present does not automatically mean it will be used up before all of the HCl is consumed. This is a limiting reagent problem.
In order to find out which reactant is the limiting reagent, you have to compare them to each other. This comparison must be done in moles, therefore, the next step will be to convert each of the grams of reactants to moles:
Again, you should not jump any conclusions about which reactant is the limiting reagent. Just because there are fewer mols of magnesium hydroxide does not mean it is the limiting reagent. Arbitrarily pick one of these reactants and calculate how many mols of the other reactant is needed to completely use up the reactant picked. In this case, magnesium hydroxide is arbitrarily chosen:
Compare the mols HCl needed to the actual mols HCl available. In this case, 1.74 mol of HCl is needed and 1.23 mol HCl is available--that's not enough. So, even though it appears that there are more mols of HCl than Mg(OH)2, the HCl is the limiting reagent. The HCl will be run out before the magnesium hydroxide and thereby limit the amount of product formed. For this reason, use the mols of HCl to calculate the theoretical yield of magnesium chloride:
The theoretical yield is the maximum amount of product which can be produced (in an ideal world). In the "real" world it is difficult to produce the amount obtained for the theoretical yield. A percent yield is often used to show how close to ideality one has obtained in a chemical synthesis. Suppose in the reaction discussed a chemist actually obtained 55.4 g of MgCl2. This is called the actual yield and would be given to you in the problem. To calculate the percent yield:
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