Calculate mass, concentration and pH of solutions

In this tutorial, we are going to do some problems which contains relationship of mass, concentration and pH in solutions. You can understand how to apply those equations in such problems to do these type of questions easily.



Question 1

Calculate the NaOH weight required to prepare 500ml of NaOH solution of pH=13. Room temperature is 250C.

Calculate concentration of solution of known pH

Calculate pOH from pH by using the pH + pOH = 14 (at 250C)
Then you can calculate the concentration of NaOH solution by pOH = -log10[OH-(aq)] .
Then find the required moles of NaOH by the equation of C =n/v . Here C = concentration, n=required moles, v = volume of solution
Now weight is measured by multiplying number of moles and molar mass.


Assumptions

The OH- received from water dissociation is negligible when it compares with OH- received from NaOH.


calculate weight concentration of solution

Calculate pOH from pH value

pH + pOH = 14
13 + pOH = 14
pOH = 1


Calculate concentration (in mol dm-3) of solution

Now we know the pOH of NaOH solution. Use the relation of pOH and OH- concentration to calculate the concentration of OH-.

pOH = -log10[OH-(aq)]
1 = -log10[OH-(aq)]
[OH-(aq)] = 0.1 mol dm-3


Find the required moles of NaOH

Concentration, moles amount, and volume relation is used.

C = n/v
0.1 = n/0.5      [500 ml = 0.5 dm3]
n = 0.05mol

NaOH amount = 0.05 mol


Find the required weight(g)

Multiply required moles times molecular weight

m = mass
n = moles
M = molecular weight
n = m/M

m = molecular weight * moles
m = M*n
m = 0.05 * 40
m = 2g



How to calculate mass given ph and volume of a solution?

Ok. First we should know, what are the known parameters of the given solution. In this example, pH and volume are given as data.

When pH value is known, concentration of H3O+ or OH- can be foundfrom pH or pOH equation.

According to the H3O+ or OH- concentration and stoichiometric ratio of dissociation of the compound, the concentration of dissolved compound is found.

With found concentration and volume of solution, dissolved amount is found.

Amount = concentration * volume

Next, we use relationship of amount, mass and molecular mass.

Amount = mass / molecular mass

Because we know the amount and molecular mass, dissolved mass can be found.



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