If given 0.090 moles of sodium sulfate in 12 mL of solution, what is the concentration?

In order to find a solution's molar concentration, or molarity, you need to determine how many moles of solute, which in your case is sodium sulfate, #"Na"_2"SO"_4#, you get in one liter of solution.

That is how molarity was defined -- the number of moles of solute in one liter of solution.

So, you know that you have #0.090# moles of solute in #"12 mL"# of solution. Your goal here will be to scale up this solution by using this information as a conversion factor to help you determine the number of moles of solute present in

#"1 L" = 10^3"mL"#

of solution. Convert the volume from milliliters to liters first

#12 color(red)(cancel(color(black)("mL"))) * "1 L"/(10^3color(red)(cancel(color(black)("mL")))) = "0.012 L"#

If #"0.012 L"# of solution contain #0.090# moles of solute, it follows that #"1 L"# will contain

#1 color(red)(cancel(color(black)("L solution"))) * ("0.09 moles Na"_2"SO"_4)/(0.012color(red)(cancel(color(black)("L solution")))) = "7.5 moles Na"_2"SO"_4#

So, if #"1 L"# of solution contains #7.5# moles of solute, it follows that the solution's molarity is

#"molarity" = color(green)(|bar(ul(color(white)(a/a)color(black)("7.5 mol L"^(-1) = "7.5 molar" = "7.5 M")color(white)(a/a)|)))#

The answer is rounded to two sig figs.