Question

How many grams of `NaOH` are contained in 200 mL of a 0.400 M solution?

Answer 1

Approx. `3*g`

Explanation:

By definition....`"concentration"="moles of solute"/"volume of solution"`...and thus concentration has units of `mol*L^-1`..

And following this definition dimensionally, `"moles of solute"="concentration"xx"volume"`..

And so here, this product gives...

`200*mLxx10^-3*mL*L^-1xx0.400*mol*L^-1=underbrace(0.0800*mol)_"moles of solute"`..

And the associated mass is `0.0800*molxxunderbrace(40.00*g*mol^-1)_"molar mass of NaOH"=??*g`

This is why chemists and physicists use units in their numerical calculations; not to confuse students, but to alert them when they have ballsed up a question. If the units didn't simplify to `mol` and `g`, we would know we had made an error...