Some ACT Science questions ask about the hypothesis each experiment is based on. Some will ask you to weaken and some will ask you to strengthen these hypotheses. For the weaken questions, you must fully understand the set-up behind the experiments in order to know whether the results will weaken a conclusion.
The process of titration is an important tool when trying to determine the concentration of an unknown. In order to use titration, one must know the concentration of one of the reactants as well as the volume of both reactants. Normally, a color change will indicate the end point of the titration. An experiment was set up as shown in Figure 1.
In Buret A is placed 15.0 ml of 0.2M H2SO4 solution. In Buret B is placed 50.0 ml of a potassium hydroxide (KOH) solution of unknown concentration. The sulfuric acid is allowed to run into the Erlenmeyer flask and to the flask three drops of phenolphthalein are added. The base is added drop by drop until the phenolphthalein turns a very faint pink color. This is considered the end point of the titration. The experiment is repeated three more times using other concentrations of potassium hydroxide (KOH). The results of this experiment are summarized in Table 1.
A second titration was done with a setup similar to that in Experiment 1. In this case, an iodine solution is placed in Buret A and various fruit juices are placed in Buret B. Once the fruit juice was placed in the flask, several drops of a starch solution were added. This solution would turn a blue-black color once the endpoint of the titration was reached.
The chemical reaction is as follows:
Ascorbic Acid + I2 → 2I- + dehydroascorbic acid.
The results of this experiment are summarized on Table 2.
One of the students doing Experiment 2 hypothesized that orange juice has the most Vitamin C of all the juices tested. Assuming that using more juice indicates the presence of more Vitamin C, is this hypothesis supported from the experiment?
Since the total volume used is the difference between the starting and ending volumes, the orange juice actually has the least Vitamin C as 50.0 – 43.8 = 6.2. According to these results, the apple juice has the most Vitamin C. The answer is (A).