In this qn, you use the mole formula,
amount (mol) = mass (g) ÷ molar mass
(molar mass is the mass number of an element, which u can find by referring to the periodic table)
so by subbing in the values given to you, you can find the answer
5. amount of Cu = 55.9 ÷ 63.5 = 0.880mol (3sf)
6. amount of N2 = 340 ÷ 2(14.0) = 12.1mol (3sf)
(note: you multiply the molar mass of N by 2 as there are 2 N atoms)
for qn 7 and 8, youre finding the mass and not the amount. By bringing the molar mass over to the other side of the formula, you get
mass = amount (mol) × molar mass
thus,
7. mass of Fe = 0.063 × 55.8 = 3.52g (3sf)
8. mass of H2 = 94.7 × 2(1.0) = 47.4g (3sf)
extra things to take note of:
- likewise, you can also find the molar mass of an element by using
molar mass = mass(g) ÷ amount(mol)
^^ this is commonly only needed when you need to find the molar mass of an isotope. the question will give you the amount and mass of the element, and you just need to factor them in to find your answer.
- typically, you use 3 sf in your final answer and 5sf in your working. remember to write (3sf) at the end of your number statement when necessary.
- when you get an answer of (eg.) 0.6mol, you still need to write it in 3sf, so your answer would be 0.600mol
