Passage V
Liquid H2O can be broken down into hydrogen gas（H2）and oxygen gas (O2) by electrolysis according to the following chemical equation:
2H2O → 2H2 + O2
A scientist performed an experiment to study the electrolysis of H2O using electricity generated from sunlight.
Experiment
Steps 1-5 were performed daily for 12 months:
1. A tank fitted with 2 electrodes一an anode (where O2 would be produced) and a cathode (where H2 would be produced)—was assembled. Each electrode was suspended in an inverted plastic tube, and each tube was marked to allow gas volume to be measured.
2. Four liters (4.0 L) of a 25% by mass aqueous solution of sodium hydroxide (NaOH) was added to the tank. As a result, the tubes were completely filled with the solution.
3. At 8:00 a.m., a rectangular solar cell was attached to the electrodes and placed next to a particular south-facing window for 8 hr. (Figure 1 shows the apparatus at the initiation of electrolysis.)
4. Eight hours later, the solar cell was detached from the electrodes, and the amount of H2 that had been produced was measured.
5. The tank, tubes, and electrodes were cleaned and dried for reuse.
Figure 2 shows the total volume of H2 produced (in L) in each month of the experiment. Table 1 shows the average solar irradiance (power per unit area), in watts per square meter (W/m2), at the location of the solar cell during each month of the experiment.
Figures and table adapted from Sergii Bespalko, "Hydrogen Production by Water Electrochemical Photolysis Using PV-Module. " @2011 by Sergii Bespalko.
29. Consider the percent by mass of NaOH in the solution added to the tank in Step 2. Approximately what mass of NaOH was in 200 g of this solution?
Your Answer is
Correct Answer is B
Explanation
All experimental procedures found that 25% sodium hydroxide was used, that is to say, 50g of 200g is sodium hydroxide