# In some acid, the mass fractions of elements are equal: hydrogen 1.25%, phosphorus 38.75%

In some acid, the mass fractions of elements are equal: hydrogen 1.25%, phosphorus 38.75%, oxygen 60%. Find the formula for this acid.

Let us denote: a is the number of hydrogen atoms, b is the number of phosphorus atoms, c is the number of oxygen atoms, M is the molecular weight of the acid.
The relative atomic mass of hydrogen is 1, phosphorus is 31, oxygen is 16.
Let’s compose the following expressions:
0.0125 = 1 * a / M;

0.3875 = 31 * b / M;

0.6 = 16 * c / M;

Let us express the molecular weight from each expression:
M = a / 0.125 = 80a;

M = 31 * b / 0.3875 = 80b;

M = 16 * s / 0.6 = 26.667s;

Let’s equate the results obtained: 80a = 80b = 26.667c. This means that a = 1, b = 1, c = 3. The resulting acid has the formula: HPO3.
Answer: The acid formula is HPO3.

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