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?

1 Answer
Stefan V.
Dec 25, 2016

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.

Related questions
See all questions in Molarity
Impact of this question
15146 views around the world
You can reuse this answer
Creative Commons License