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.
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.
The OH- received from water dissociation is negligible when it compares with OH- received from NaOH.
pH + pOH = 14
13 + pOH = 14
pOH = 1
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
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
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
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.