How to find the number of atoms of an element in $x$ amount of a compound

Question

Lets say I am given a certain mass of a compound. Additionally, the number of moles of the compound itself are known, as well as its molar mass. How can I, using an equation, find the number of atoms (in moles) of an element in said compound?

After trying myself, I derived the equation, $A = \frac{pm}{a}$, where $A$ is the number of atoms, $p$ is the percent composition, $m$ is the mass of the compound, an $a$ is the AMU measure of the element. Logically, it seems this would work. However, it does not algebraically. This is because $p = \frac{a}{M}$, where $M$ is the AMU measure of the compound. If you factor that in, you can see that both $a$'s cancel each other out, so I end up with the same answer for any element of the same compound.

What can I do to prevent this from happening?

Answer 1

If $m$ is the mass of the compound in grams, and $M$ is the AMU mass of the compound, there are $N_A \frac mM$ molecules of the compound, where $N_A$ is Avogadro's number. The mass of your atoms is then $mp$ and the number of atoms is $A=N_A\frac {mp}a$

Answer 2

If $m$ is the mass of an element in grams, and $M$ is the AMU mass of the element, then the number of atoms is given by $N_A \frac{m}{M}$, where $N_A$ is Avogadro's number.

The chemical formula for the compound tells you the ratio of elements in the compound. Let's say we are talking about water. Hydrogen is 1, Oxygen is 16. Thus in every 18 kg of water, there is 16 kg of oxygen and 2 kg of hydrogen. Let's say you had 26 kg of water. Then we have 4 kg of hydrogen. Then just plug this into the equation above for the answer.