Question

You have 2.5L of .14M salt solution, how many grams of salt would be needed to make this solution?

Answer 1

`"20. g"`

Explanation:

The thing to recognize here is that the molarity of the solution tells you the number of moles of solute present in exactly `"1 L"` of this solution.

In your case, the solution is said to have a molarity of `"0.14 M"`, which implies that every `"1 L"` of this solution contains `0.14` moles of salt, the solute.

Now, in order to have `"2.5 L"` of `"0.14 M"` salt solution, you need the solution to contain

`2.5 color(red)(cancel(color(black)("L solution"))) * "0.14 moles salt"/(1color(red)(cancel(color(black)("L solution")))) = "0.35 moles salt"`

In order to find the number of grams needed to have `0.35` moles of salt, you need to use the compound's molar mass. Assuming that the salt is table salt, or sodium chloride, `"NaCl"`, you will have

`0.35 color(red)(cancel(color(black)("moles NaCl"))) * "58.4 g"/(1color(red)(cancel(color(black)("mole NaCl")))) = color(darkgreen)(ul(color(black)("20. g")))`

The answer must be rounded to two sig figs, the number of sig figs you have for your values.