You want to test how the mass of a reactant affects the speed of a reaction.

Which of the following is an example of a controlled experiment to test this?

See below.

Explanation:

Although the examples have not been included, we can give an explanation and creatively write some hypothetical examples of a controlled experiment to test how the mass of a reactant affects the speed of a reaction.

Several conditions affect the speed of a reaction, such as the concentration of each reactant, the temperature, the contact surface area, and the use of a catalyst.

Then, when you want to test how the mass of a reactant affects the speed of a reaction you should perform experiments where only the mass of the reactant that you are testing changes. This is, you have to perform a controlled experiment where the mass of the other reactants, the volume of the reactors, the temperature, the size of the reactants, and the use or not use of catalyst, are the same in all the tests. This is, you must assure that only the mass of the reactant that you want to test, i. e. your independent variable, change.

For such experiment, then, your conditions are:

Indepentend variable (the variable that you manipulate) : the mass of reactant.

Dependent variable: the speed of reaction (which shall change when you change the mass of reactant.

Controlled variables: light (which is a possible catalyst), temperature, mass of other reactants, volume (which affects the concentration), size of the reactants (which affects the contact surface area).

An hypothetical example, that meet those conditions might be:

In an experiment different amounts of calcium bicarbonate, carefully weighted, are added to a fixed amount of vinegar (diluted acetic acid) of fixed composition, under the same conditions of temperature and light Then the amounts of CO₂ gas produced were measured and recorded.