The percentage of MgO in cement is calculated using :
- AWeight of residue × 32·4
- BWeight of residue × 27·4
- CWeight of residue × 42·7
- DWeight of residue × 72·4Correct
Solution
Concept
Magnesia determination (as magnesium pyrophosphate)
Formula
MgO % = (W × 0.3622 / sample mass) × 100
Step-by-step solution
1. Magnesium is weighed as magnesium pyrophosphate Mg₂P₂O₇. 2. MgO / Mg₂P₂O₇ = 2 × 40.3 / 222.6 = 0.3622. 3. For a 0.5 g sample: MgO % = W × 0.3622 × 100 / 0.5 = W × 72.4.
Explanation
The IS method weighs the pyrophosphate residue and converts to MgO using the factor 0.3622. For the 0.5 g sample used in the test this becomes a multiplier of 72.4.
Why the other options are wrong
32.4, 27.4 and 42.7 do not correspond to 0.3622 × 100 / (sample mass).
Shortcut
0.3622 × 100 / 0.5 = 72.4.
Common mistake
Forgetting to account for the 0.5 g sample.
Correct answer: D) Weight of residue × 72·4