In chemistry, aqueous solutions are prepared for various purposes. As examples, acidic or basic aqueous solutions are prepared by dissolve a specific quantity of chemical compound. According to the concentration and volume of the required solution, weight or volume of solid chemical or concentrated solution should be measured or calculated. As well, pH value of the solution is given to calculate concentration and other parameters.
In this tutorial, we are going to practice some problems which contains relationships of mass, concentration, volume and pH in aqueous solutions. You can understand how to apply those equations in such problems to solve these type of questions easily.
Before solve this kind of equation, following steps should be followed by you.
In step 1, amount of KOH was calculated. Now, we can use the relationship of mass (m), molar mass (M) and amount (n) to calculate required mass of KOH. As an additional data, molar mass of molar mass of KOH should be calculated prior to calculate mass of KOH.
Then apply n = m/M equation.
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.