Question

Question #ed6fd

Answer 1

`"5 g"`

Explanation:

The problem tells you that your solution must have a molarity of `"0.5 mol L"^(-1)` and a total volume of `"250 mL"`, so the first thing that you need to do here is to convert the volume of the solution to liters.

`250 color(red)(cancel(color(black)("mL"))) * "1 L"/(10^3color(red)(cancel(color(black)("mL")))) = "0.250 L"`

Now, a `"0.5-mol L"^(-1)` solution will contain `0.5` moles of solute for every `"1 L"` of solution.

This means that in order for your solution to have a molarity of `"0.5 mol L"^(-1)`, it must contain

`0.250 color(red)(cancel(color(black)("L solution"))) * "0.5 moles NaOH"/(1color(red)(cancel(color(black)("L solution")))) = "0.125 moles NaOH"`

Finally, to convert the mass of sodium hydroxide to grams, use the compound's molar mass.

`0.125 color(red)(cancel(color(black)("moles NaOH"))) * "40 g"/(1color(red)(cancel(color(black)("mole NaOH")))) = color(darkgreen)(ul(color(black)("5 g")))`

The answer is rounded to one significant figure.