For part (b)
I'll show you how to solve part (b), since it's a little more interesting than part (a).
So, you know that you're dealing with a solution of potassium dihydrogen arsenate,
You also know that molairty is defined as moles of solute, which in you case is potassium dihydrogen arsenate, divided by liters of solution.
#"molarity" = "moles of solute"/"liters of solution"" "#, or #" "C = n_"solute"/V_"solution"#
So, you need to pick out a volume of this
Notice that they give you the mass of the solute instead. This means that you're going to have to calculate how many moles of potassium dihydrogen arsenate you ahve in that many grams of the compound.
To do that, use its molar mass, which tells you what the mass of one mole of a substance is
#8.10color(red)(cancel(color(black)("g"))) * ("1 mole KH"_2"AsO"_4)/(180.0334color(red)(cancel(color(black)("g")))) = "0.04492 moles"#
Now you know how many moles of solute you need to get from the sample. Rearrange the equation to solve for the volume that would contain this many moles
#C = n/V implies V = n/C#
#V = (0.04492color(red)(cancel(color(black)("moles"))))/(0.0500color(red)(cancel(color(black)("moles")))/"L") = "0.8984 L"#
#V = color(green)("0.898 L")#