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

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Discussion Overview

The discussion revolves around calculating the percentage by mass of phosphorus in a fertilizer that contains 30.0% phosphorus pentoxide (P2O5) soluble in water. Participants are exploring the relationship between the mass percentage of P2O5 and the mass percentage of phosphorus (P) within it, focusing on a homework problem from an AS level multiple-choice question.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant states that the percentage by mass of phosphorus in P2O5 is calculated as 68/148 X 100, resulting in 45.9%.
  • Another participant confirms that the fertilizer contains 30% by weight of phosphorus pentoxide.
  • A subsequent post suggests a calculation method to find the mass of phosphorus based on the 30% P2O5 content and the previously calculated percentage of phosphorus.
  • A later reply challenges the accuracy of the 45.9% figure, indicating that it is not the correct percentage of phosphorus in P2O5.

Areas of Agreement / Disagreement

Participants express differing views on the correct percentage of phosphorus in P2O5, with some calculations being challenged. The discussion remains unresolved regarding the accurate percentage of phosphorus.

Contextual Notes

There is an unresolved issue regarding the correct calculation of the percentage of phosphorus in P2O5, as well as the implications of the 30% P2O5 content in the fertilizer.

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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%
 
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Good start. Now, there is 30% by weight phosphorus pentoxide in the fertiliser in total...
 
Code:
sjb-2812 said:
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%?
 
Not bad, the only problem being - there is no 45.9% P in P2O5. Close, but different number.
 

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