Question

How many moles of hydrogen are in 5.2 moles of C7H18

Answer 1

In one mole of C7H18 there are 18 moles of H (the number folowing the H)*
>> the ratio is 1:18

In 5.2 moles of C7H18 there are x moles of H
>> the ratio is 5.2:x

Cross multiply the two ratios
1x = 18×5.2
x = 93.6 moles of H

>> In 5.2 moles of C7H18 there are 93.6 moles of H




* This isnt a rule that you can always use.
However to find the mole of a certain element in a certain molucle all you have to do is count how many moles of the element are present in the molecule.
>> example1 >> H2O ;
2 H and 1 O

>> example2 >> CH3COOH ; [you add up all the moles of the same element]
(1+1) 2 C , (3+1) 4 H and (1+1) 2 O

>> example3 >> Mg(OH)2 ; [you multiply whetever is in parenthesis by the number after it 2] 1 Mg , (1×2) 2 O and (1×2) 2 H

Answer 2

Answer: 93.6 moles

Explanation:

According to avogadro's law, 1 mole of every substance weighs equal to the molecular mass and contains avogadro's number $6.023\times 10^{23}$ of particles

1 molecule of $C_7H_{18}$ contains 18 atoms of hydrogen

1 mole of $C_7H_{18}$ contains= $18\times 6.023\times 10^{23}=108.4\times 10^{23}$ atoms of hydrogen.

5.2 moles of $C_7H_{18}$ contains= $108.4\times 10^{23}}{1}\times 5.2=563.7\times 10^{23}$ atoms of hydrogen.

Now moles of hydrogen=$\frac{\text {given atoms}}{\text {avogadros number}}=\frac{563.7\times 10^{23}}{6.023\times 10^{23}}=93.6moles$ of hydrogen atom.

Thus 5.2 moles of $C_7H_{18}$ contains 93.6 moles of hydrogen.