What is the percentage by mass of phosphorus in the fertiliser? AS level MCQ

[mutineer123]

Homework Statement

A garden fertiliser is said to have a phosphorus content of 30.0% ‘P
2
O5
soluble in water’.
What is the percentage by mass of phosphorus in the fertiliser?
A 6.55% B 13.1 % C 26.2% D 30.0%

Homework Equations

The Attempt at a Solution

I got the percentage by mass of P in P2O5, which was 68/148 X 100 which was 45.9%

 

[sjb-2812]

Good start. Now, there is 30% by weight phosphorus pentoxide in the fertiliser in total...

 

[mutineer123]

Good start. Now, there is 30% by weight phosphorus pentoxide in the fertiliser in total...

KK so can you just see if I am right? Let's say the compund is P2O5X. Out of the compund P2O5 is 30%. Now out of this 30% P is 45.9%. So amount of P is 45.9/100 X 30%?

 

[Borek]

Not bad, the only problem being - there is no 45.9% P in P₂O₅. Close, but different number.

 

[DeeNos]

.

To find the percentage by mass of phosphorus in the fertiliser, we need to consider the molar mass of P2O5 and the molar mass of phosphorus. P2O5 has a molar mass of 141.94 g/mol, while phosphorus has a molar mass of 30.97 g/mol.

Therefore, the mass percentage of phosphorus in the fertiliser can be calculated as (30.97 g/mol / 141.94 g/mol) x 100 = 21.8%.

This means that 21.8% of the fertiliser, by mass, is composed of phosphorus. The rest of the percentage is made up of other components such as oxygen and other nutrients.

Therefore, the correct answer is C) 26.2%. This is because the given percentage of 30.0% P2O5 is soluble in water, which means that only a portion of the total P2O5 is actually phosphorus. By converting the percentage of P2O5 to the percentage of phosphorus, we get 26.2%.