Calculate the molarity of the solute in a solution containing 14.2 KCl in 250 mL solution?

I would say that you're dealing with a solution that contains #14.2# grams of potassium chloride, #"KCl"#, in #"250 mL"# of solution.

If this is the case, your strategy here will be to use the molar mass of potassium chloride to calculate how many moles are present in the sample

#14.2 color(red)(cancel(color(black)("g"))) * "1 mole KCl"/(74.55color(red)(cancel(color(black)("g")))) = "0.1905 moles KCl"#

Now, molarity is simply a measure of a solution's concentration in terms of how many moles of solute it contains per liter of solution.

This means that in order to find a solution's molarity, you essentially must figure out how many moles of solute you have in #"1 L"# of solution.

In your case, you know that #"250 mL"#, which is equivalent to #1/4"th"# of a liter, contains #0.905# moles.

All you have to do now is scale up this solution so that its volume becomes #"1 L"#. Simply put, if #1/4"th"# of a liter contains #0.1905# moles, it follows that #"1 L"# will contain four times as many moles of solute.

#1 color(red)(cancel(color(black)("L solution"))) * "0.905 moles KCl"/(1/4color(red)(cancel(color(black)("L solution")))) = "0.76 moles KCl"#

So, if #"1 L"# of this solution contains #0.76# moles of solute, it follows that its molarity is

#"molarity" = color(green)(|bar(ul(color(white)(a/a)color(black)("0.76 mol L"^(-1))color(white)(a/a)|)))#

The answer is rounded to two sig figs, the number of sig figs you have for the volume of the solution.