Which of these is not a factor that may affect percentage yield? (A) The purity of the reactants, (B) unwanted products formed from side reactions, (C) the loss of product during separation or handling, (D) the reaction being reversible, (E) the amount of gas produced.
When a chemical reaction occurs and reactants are transformed into products, the amount of product that forms is called the yield. The yield can be expressed in terms of a mass of product or volume of product or moles of product. We can calculate a yield value, and we can measure the yield. When we calculate the value, we are determining a theoretical yield. This value is the maximum possible amount of product obtained from a chemical reaction if all the reactants are converted to products.
We calculate theoretical yield using the amounts of the reactants, mass, volume, or moles, and the molar ratio in which the reactants react with each other and in which the products form, which we get from the stoichiometric coefficients from the balanced equation. So again, theoretical yield is calculated using this information.
In reality, however, the yield that is actually obtained in the lab by measuring is usually a slightly different value to the theoretical or calculated value. We call this measured yield the actual yield, which is the amount of product experimentally obtained from carrying out a chemical reaction. If we take the actual yield value measured after carrying out a reaction and divide it by the calculated theoretical yield and convert to a percentage, we get the percentage yield, which is what the question is asking us about.
If we look at answer option (E) the amount of gas produced, we see that this is a measured value. When we measure the amount of product, in this case a gas product, we are measuring the actual yield. And this value we put into the equation with a theoretical yield to determine a percentage yield. However, answer options (A), (B), (C), and (D) are factors or processes which would affect the actual yield. In other words, they would affect the amount of a gas produced. And if the actual yield is influenced by some factors, then the percentage yield will also be influenced. The question asks, “Which is not a factor that affects percentage yield?” And we can see that the answer is (E) the amount of gas produced.
Let’s confirm this by ruling out the other four answer options.
The purity of the reactants will influence the amount of product which forms. If there is an impurity in the reactants, then the measured mass of the reactants, which we could use to calculate theoretical yield, is not accurate. This measured mass includes the mass of impurity, which will not go on to form product. The presence of impurities decreases the actual yield, which in turn decreases the percentage yield. The purity of the reactants is a factor that affects percentage yield. Highly pure reactants tend to increase the actual yield and therefore increase the percentage yield.
If a side reaction occurs — that is, if other unwanted or undesired products also form from the reactants — then the actual yield of the desired product will decrease, decreasing the percentage yield. Unwanted products formed from side reactions is a factor that may affect percentage yield.
The actual yield, and therefore in turn the percentage yield, might also be influenced by the loss of product during separation or handling. There are many ways in which product can be lost during the course of a reaction or purification. Here is one example. If a solid product is separated from a reaction mixture by filtration and washed with a solvent, small amounts of it may redissolve if the product is soluble in that solvent. And some product may end up in the filtrate. Some of the product from the filter paper is lost.
When this product is dried and weighed, the value for the actual yield will be lower than expected because of this loss of product. And this in turn will influence the percentage yield. Loss of product during separation or handling is a factor that affects percentage yield.
Lastly, when a reaction is reversible, some of the products react with each other to reform the reactants. This decreases the amount of product obtained at the end of the reaction, and this affects the percentage yield. So we can also rule out answer option (D), since a reversible reaction is a factor that affects percentage yield.
In fact, there are many factors and experimental errors which influence the actual yield and therefore the percentage yield. A percentage yield is unlikely to be exactly 100 percent. In fact, it is often less than 100 percent and only occasionally more than 100 percent.
The question asked, “Which of these is not a factor that may affect percentage yield?” The answer is (E) the amount of gas produced.