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# Where is this coming from Watch

1. Question
A has percentage composition by mass, 43.7% of P and 56.3% of O
Mr is 284

Mark scheme

P : O = 43.7/31 : 56.3/16 / 1.41 : 3.52
Ratio P:O = 2 : 5 / Empirical formula = P2O5
Molecular formula = P4O10 (from Mr value)
Ca3(PO4)29

I get the answer 1.41 : 3.52
But Where has the 2:5 come from?
I know you divide by the smallest value, but when I do that, I get
1.41/1.41 = 1 and 3.52/1.42 = 2.3
2. (Original post by ILoveUSA)
Question
A has percentage composition by mass, 43.7% of P and 56.3% of O
Mr is 284

Mark scheme

P : O = 43.7/31 : 56.3/16 / 1.41 : 3.52
Ratio P:O = 2 : 5 / Empirical formula = P2O5
Molecular formula = P4O10 (from Mr value)
Ca3(PO4)29

I get the answer 1.41 : 3.52
But Where has the 2:5 come from?
I know you divide by the smallest value, but when I do that, I get
1.41/1.41 = 1 and 3.52/1.42 = 2.3
3.52/1.41 = 2.496
3. (Original post by charco)
3.52/1.41 = 2.496
That still doesn't give the 2:5 ratio, like the mark scheme has
4. (Original post by ILoveUSA)
That still doesn't give the 2:5 ratio, like the mark scheme has
It does. You have the ratio 1:2.49... which is essentially, 1:2.5

Multiply by 2 and you get 2:5

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