Question #ed6fd

1 Answer
Sep 30, 2017

"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.