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.

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