Question #9609d

Your goal here is to find the mass of sodium chloride that would be required to make #"200 mL"# of a #"10 mM"# solution, so focus on finding the number of moles of sodium chloride first.

Now, a #"10 mM"# solution contains #10# millimoles of solute for every #"1 L"# of solution. Since you know that

#"1 mol" = 10^3"mmol"" "# and #" " "1 L" = 10^3"mL"#

you can say that #"200 mL"# of solution will contain

#200 color(red)(cancel(color(black)("mL"))) * (1color(red)(cancel(color(black)("L"))))/(10^3color(red)(cancel(color(black)("mL")))) * (10 color(red)(cancel(color(black)("mmoles NaCl"))))/(1color(red)(cancel(color(black)("L solution")))) * "1 mole NaCl"/(10^3color(red)(cancel(color(black)("mmoles NaCl"))))#

#= " 0.0020 moles NaCl"#

Now all you have to do is use sodium chloride's molar mass to convert the moles to grams

#0.0020 color(red)(cancel(color(black)("moles NaCl"))) * "58.44 g"/(1color(red)(cancel(color(black)("mole NaCl")))) = color(green)(bar(ul(|color(white)(a/a)color(black)("0.1 g")color(white)(a/a)|)))#

The answer is rounded to one significant figure.