Question

How do you calculate the number of grams of substance needed to make the following solution: `50` `"cm"^3` of `"NaOH (aq)"`, concentration `2` `"mol"` `"dm"^-3`?

Answer 1

Here's how you can do that.

Explanation:

Start by calculating how many moles of sodium hydroxide you need in order to have `"50 cm"^3` of `"2 mol dm"^(-3)` solution.

As you know, molarity is defined as the number of moles of solute, which in this case is sodium hydroxide, present in `"1 dm"^3` of solution.

This means that a `"2 mol dm"^(-3)` solution will have `2` moles of sodium hydroxide dissolved for every `"1 dm"^3` of solution. In your case, the sample must contain

`50 color(red)(cancel(color(black)("cm"^3"solution"))) * (1color(red)(cancel(color(black)("dm"^3))))/(10^3color(red)(cancel(color(black)("cm"^3"solution")))) * "2 moles NaOH"/(1color(red)(cancel(color(black)("dm"^3"solution"))))`

`= " 0.10 moles NaOH"`

Now, in order to convert this to grams of sodium hydroxide, use the compound's molar mass. You will end up with

`0.10 color(red)(cancel(color(black)("moles NaOH"))) * "40.0 g"/(1color(red)(cancel(color(black)("mole NaOH")))) = color(darkgreen)(ul(color(black)("4 g")))`

The answer must be rounded to one significant figure.