The simplest and most helpful visual aid for this experiment would be if the student sets rulers of some sort or maybe pieces of wood that are properly marked with certain measuring unite. If they are set right next to each plant, the student will be able to monitor constantly how are the plants growing, and the smaller the measuring unites the better, as the student will be able to monitor the situation in higher detail.
Over time, the plants will most surely develop differently, so the student will have it easier to notice the changes. Considering that it is an experiment in question, the student should use proper measurements and to be able to have detailed numbers for it.
Answer:
the vapor pressure of this solution would increase if some of the water were allowed to evaporate
As mentioned above, phosphoric acid has 3 pKa values, and after 3 ionization it gives 3 types of ions at different pKa values:
H₃PO₄(aq)
+ H₂O(l) ⇌ H₃O⁺(aq) + H₂PO₄⁻ (aq) pKₐ₁
<span>
</span>H₂PO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HPO₄²⁻ (aq) pKₐ₂
HPO₄²⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + PO₄³⁻ (aq) pKₐ₃
At the highest pKa value (12.4) of phosphoric acid, the last OH group will lose its hydrogen. On the picture I attached, it is shown required protonated form of phosphoric acid before reaction whose pKa value is 12.4.
The relationship between pressure and solubility of the gas is given by Henry's law as:

where,
is the solubility of the gas.
is proportionality constant i.e. Henry's constant.
is pressure of the gas.
(given)
(given)
Substituting the values,

To convert
to
:
Molar mass of benzene,
= 

Now for converting into
:
Since, 
So,
.
Hence, the solubility of benzene in water at
in
is
.