AS chemistry

Find the number of moles in 0.207g of lithium. Lithium exists as 2Li (as it is a chemical equation) and its given in the question that the Ar of Li = 6.9. The answer is 0.03. Shouldn't the answer be 0.015 because as it is 2Li the Mr will be 6.9 × 2 = 13.8. So 0.207÷13.8 = 0.015. I know I'm wrong somewhere so can anyone help?.

You only use the 2 in front of lithium when working out other moles in the equation for example if the equation was 2Li + 2H20 = 2LiOH + H2 and you were to find the moles of hydrogen. You would divide the moles of lithium by 2 to find hydrogen (H2) because of the 2:1 molar ratio. So, you dont need to multiply the ar of lithium regardless of the number in front of it. Hopefully that made sense.

When you’re working out the mr of an element or molecule you don’t look at the number before. The number before is used to work out molar ratio, you’ll probably learn about molar ratio soon .

Okay. For example the number of moles in 2.5g of 2NaOH is 0.03125. Right?

No. The mr of NaOH is 23+16+1. Then you do 2.5/40. The number before the element doesn’t affect the mr