Chemistry in uni

Question

Hey!

A 10.0% solution (w / w) of NaOH (aq) has a density of 1.11 g / mL. What is the concentration measured in mol/L?

Thanks for helping me out :)

Answer 1

Hi Shahzaib,

This question will just require some simple conversions.

The first point to note is that when a concentration is described as (w/w) this stands for weight per weight. This means that the concentration given is telling you what mass of solute is present per weight of solution.

In other words, the concentration provided here (10.0% w/w) is telling you that for every 100 grams of solution, there are 10 grams of NaOH and 90 grams of water.

First we will convert the mass of NaOH to the moles of NaOH by dividing the mass of NaOH present in solution by the molar mass of NaOH. The molar mass of NaOH is 39.996 g/mol. Therefore the equation will be:

(10.0 grams NaOH)/(39.996 g/mol) = 0.250 moles NaOH

Now that we know how many moles of NaOH are present in solution, we need to determine the total volume of solution.

Given that we know the density of the solution to be 1.11 g/mL we can determine how many mL of solution are represented by 100 grams of solution. To do this we simply divide the total mass of solution (100 grams) by the given density to determine the volume (in mL). This equation will be:

(100 grams solution)/(1.11 g/mL) = 90.1 mL of solution (remember your sig figs)

So lets review what we know. We know that in 100 grams of solution, we have 0.250 moles of NaOH. We also know that, according to the given density, that 100 grams of solution will equate to a volume of 90.1 mL or 0.0901 L. Therefore, to solve for the concentration in mol/L we simply need to divide the number of moles of NaOH in 100 grams of solution by the volume of 100 grams of solution. Therefore the answer will be:

(0.250 moles NaOH)/(0.0901 L) = 2.77 mol/L NaOH