Question

How many grams of NaCl are in 225 mL of a 0.75 M NaCl solution? (The molecular weight of NaCl is 58.44 g.)

Answer 1

`"9.9 g NaCl"`

Explanation:

The first thing you need to do here is to use the molarity of the solution to determine how many moles of sodium chloride you have in your sample.

As you know, molarity tells you the number of moles of solute, which in your case is sodium chloride, present in `"1 L"` of solution. This means that a `"0.75 M"` sodium chloride solution will contain `0.75` moles of sodium chloride for every `"1 L"` of solution.

`"0.75 M: " "0.75 moles NaCl " stackrel( color(white)(a) color(blue)("for every") color(white)(aaa) )(rarr) "1 L of solution"`

You can thus use the molarity of the solution as a conversion factor to find the number of moles of sodium chloride present in your sample

`225 color(red)(cancel(color(black)("mL"))) * (1 color(red)(cancel(color(black)("L"))))/(10^3color(red)(cancel(color(black)("mL")))) * "0.75 moles NaCl"/(1color(red)(cancel(color(black)("L solution")))) = "0.16875 moles NaCl"`

To convert this to grams, use the molar mass of sodium chloride

`0.16875 color(red)(cancel(color(black)("moles NaCl"))) * "58.44 g"/(1color(red)(cancel(color(black)("mole NaCl")))) = color(darkgreen)(ul(color(black)("9.9 g")))`

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