# Question ed6fd

Sep 30, 2017

$\text{5 g}$

#### Explanation:

The problem tells you that your solution must have a molarity of ${\text{0.5 mol L}}^{- 1}$ and a total volume of $\text{250 mL}$, 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 ${\text{0.5-mol L}}^{- 1}$ solution will contain $0.5$ moles of solute for every $\text{1 L}$ of solution.

This means that in order for your solution to have a molarity of ${\text{0.5 mol L}}^{- 1}$, 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.

$0.125 \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{moles NaOH"))) * "40 g"/(1color(red)(cancel(color(black)("mole NaOH")))) = color(darkgreen)(ul(color(black)("5 g}}}}$

The answer is rounded to one significant figure.