1mol of hydrogen gas and 1 mol of iodine vapour were mixed and allowed to react. after t seconds 0.8 mol of hydrogen remained.
the number of miles of hydrogen iodide formed at t seconds was..
a) 0.2
b) 0.4
c) 0.8
d) 1.6
the answer is b but explain how you work this out?

At the beginning there was just 1 mole of hydrogen and 1 mole of iodin.
The reaction goes:

One mole of hydrogen goes with one mole of iodin, creating 2 moles of product.
If 0.8 of hydrogen remaind, it means that 1-0.8=0.2 reacted.
So:
1 mole H2 react with 1 mole I2 creating 2 moles of products.
So 0.2 of hydrogen reacted with 0.2 mole of iodin creating 0.4 moles.