Finding moles of one element in a compound

[Amanda 3H]

I'm having some trouble finding the moles of one specific element in a compound. For example, in homework problem E23b, find moles of SO3 in 7x10^2mgSO3. First, I converted 7x10^2mgSO3 into grams and got .7gSO3. Then I divided .7gSO3 by its molar mass, 80.066 and got .0874mol SO3. Is this method correct? I don't feel confident in my answer but I'm not sure why.

[Isha_Maniyar_Dis2E]

Your method is correct! You converted from mg to g, and then used the ratio between grams and moles to find the moles of SO3.

However, you said you were having trouble finding the moles of one specific element. In this problem, you found the moles of the compound SO3. If you wanted to find the moles of oxygen in this problem, for example, you would need to multiply your calculated moles of SO3 by 3 moles of oxygen (because in every 1 mole of SO3, there are 3 moles of oxygen).

Hope this helped!