Without more information given, I'm going to have to make the assumption that you are looking at the percent yield when sulfuric acid is synthesized. For that, we will need a chemical reaction
One way that sulfuric acid is produced is through the reaction of sodium sulfate and hydrochloric acid.
Na2SO4 + 2HCl --> H2SO4 + 2NaCl exothermic (deltaH < 0)
For a reaction, we can change the conditions in order to increase the yield of a reaction. If we have an excess of either of the reactants, we will favor the formation of more product. Or if we remove a product we can also favor the formation of more products.
The other condition which we look at is temperature. Because this is an exothermic reaction, heat is generated. If we increase the temperature of the reaction (i.e. add heat), we will actually decrease the yield of the reaction. If we lower the temperature, we will increase the yield since the reaction is producing heat. Raising the temperature can be seen as adding product since heat is also produced from the reaction where lowering the temperature has the effect of removing product.