# 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

Here's how you can do that.

#### Explanation:

Start by calculating how many *moles* of sodium hydroxide you need in order to have

As you know, **molarity** is defined as the number of moles of solute, which in this case is sodium hydroxide, present in

This means that a **moles** of sodium hydroxide dissolved **for every** **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**.