The amounts in moles of any pair of substances in a reaction are related through the coefficients in the balanced equation as shown below.
In Step 1 the amount of known substance, n(A), is divided by a (its coefficient in the balanced equation) as above to determine the amount in moles of reaction that can occur for that amount of substance.
A titration consumes 1.5 mol NaOH.
2 mol NaOH are used for each mol of reaction.
Thus 1.5 mol/2 or 0.75 mol of reaction can occur.
In Step 2 amount of unknown substance (n(B) above) is calculated by multiplying the known amount in moles of reaction by the coefficient of the unknown substance (b) in the balanced equation.
The method can be summarised:
Step 1: Calculate amount in moles of reaction (divide the amount of known substance by its coefficient in the balanced equation).
Step 2: Calculate the amount in moles of unknown (multiply the result of Step 1 by the coefficient of the unknown substance).
For a reaction
aA + bB
cC + dD
If A is known, and B is to be calculated.
A, B,C and D are substances,
a, b, c and d are the stoichiometric coefficients
A mathematically easier method for calculation (1) is to separate it into two steps.aA + bB
cC + dDIf A is known, and B is to be calculated.
| n(A) | = | n(B) | (1) |
| a | b |
a, b, c and d are the stoichiometric coefficients
In Step 1 the amount of known substance, n(A), is divided by a (its coefficient in the balanced equation) as above to determine the amount in moles of reaction that can occur for that amount of substance.
There are 6.022 × 1023 reactions per mol of reactions.
For example:
2NaOH + H2SO4 Na2SO4 + 2H2O
Each mole of reaction requires 2 mol NaOH, 1 mol H2SO4 and produces 1 mol Na2SO4 and 2 mol H2O.
2NaOH + H2SO4
Na2SO4 + 2H2OEach mole of reaction requires 2 mol NaOH, 1 mol H2SO4 and produces 1 mol Na2SO4 and 2 mol H2O.
A titration consumes 1.5 mol NaOH.
2 mol NaOH are used for each mol of reaction.
Thus 1.5 mol/2 or 0.75 mol of reaction can occur.
In Step 2 amount of unknown substance (n(B) above) is calculated by multiplying the known amount in moles of reaction by the coefficient of the unknown substance (b) in the balanced equation.
For the example above:
If 1.5 mol NaOH used, 0.75 mol reaction occurs
n(H2SO4) used = 0.75 mol × 1 = 0.75 mol
n(Na2SO4) formed = 0.75 mol × 1 = 0.75 mol
n(H2O) formed = 0.75 mol × 2 = 1.5 mol
If 1.5 mol NaOH used, 0.75 mol reaction occurs
n(H2SO4) used = 0.75 mol × 1 = 0.75 mol
n(Na2SO4) formed = 0.75 mol × 1 = 0.75 mol
n(H2O) formed = 0.75 mol × 2 = 1.5 mol
The method can be summarised:
Step 1: Calculate amount in moles of reaction (divide the amount of known substance by its coefficient in the balanced equation).
Step 2: Calculate the amount in moles of unknown (multiply the result of Step 1 by the coefficient of the unknown substance).