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

• mutineer123
In summary, the conversation discussed the phosphorus content of a garden fertiliser, which is said to be 30.0% P2O5 soluble in water. The question was to determine the percentage by mass of phosphorus in the fertiliser, with options ranging from 6.55% to 30.0%. One attempt at solving the problem involved calculating the percentage of phosphorus in P2O5, which was found to be 45.9%. However, it was clarified that the total phosphorus content in the fertiliser is 30% by weight, and not all of it is in the form of P2O5.
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%

## 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%

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.

.

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%.

## 1. What is the meaning of "percentage by mass" in fertiliser?

The percentage by mass in fertiliser refers to the amount of a specific element (in this case, phosphorus) present in the fertiliser by weight. It is usually expressed as a percentage of the total weight of the fertiliser.

## 2. Why is the percentage by mass of phosphorus important in fertiliser?

Phosphorus is a crucial nutrient for plant growth and development. It plays a vital role in photosynthesis, energy transfer, and root development. Therefore, knowing the percentage by mass of phosphorus in fertiliser helps determine the effectiveness and suitability of the fertiliser for specific plants and soils.

## 3. How is the percentage by mass of phosphorus calculated in fertiliser?

The percentage by mass of phosphorus can be calculated by dividing the mass of phosphorus in the fertiliser by the total mass of the fertiliser, then multiplying by 100. This will give the percentage by weight of phosphorus in the fertiliser.

## 4. What is the typical percentage by mass of phosphorus in fertiliser?

The typical percentage by mass of phosphorus in fertiliser varies depending on the type of fertiliser. However, it is usually between 5-15% for most commercial fertilisers. Some specialised fertilisers may contain higher percentages of phosphorus.

## 5. How does the percentage by mass of phosphorus affect plant growth?

Having an appropriate percentage of phosphorus in fertiliser can significantly impact plant growth by promoting root development, increasing crop yield, and improving overall plant health. However, excessive amounts of phosphorus can have adverse effects on the environment and may even inhibit plant growth.

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