Question #ed6fd

1 Answer
Stefan V.
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.

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