Here it is
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According to Henry's law
partial pressure of CO₂ = K_H Xmole fraction of CO₂ in the solution.
Using this equation we can calculate mole fraction of CO₂ in the solution which has 200 ml of water Convert 200 ml  of water into 200 grams by using density of water as 1 gram per ml
Now you know the mass of water and  you know the mole fraction of CO₂ You can calculate the moles of CO₂ in the given volume of water.Convert moles of CO₂ to grams of CO₂ and  [grams of CO₂ / volume of the solution ]X 100% will be the answer

Most of CO₂ dissolved in water will form carbonic acid and carbonic acid is a weak acid will undergo dissociation to give H ⁺ ions in water which makes solution acidic
The acid dissociation constant for first proton is is more than that of second proton  so the pH will be determined by the ka1 value
pH of a weak acid = -log([HA]Ka1)^1/2
[HA] = moles /volume in L

The question looks simple but it is not. You have explained the solution very well.
Basically, this is the formula that will be used [grams of CO2 / volume of the solution ]X 100%. Find the values, convert them to appropriate units and you will get the answer for the first one.