According to this reaction: #2Al+6HBr -> 2AlBr_3 + 3H_2# If 4 moles of #Al# react with 8 moles of #HBr#, how many moles of #H_2# are formed?
You have the chemical equation, it tells you EXPLICITLY, that 2 moles of aluminum metal give 3 moles of dihydrogen gas upon reaction. You started with 4 moles of aluminum, thus
This problem was posed to introduce you to stoichiometry, which is a fancy way of saying garbage in equals garbage out. We practise stoichiometry on a smaller scale when we balance our bank accounts, or even when we make a cash or electronic transaction. The debit made to out account for goods of a certain value, must equal the credit made to the account of the vendor who sold you the goods. Likewise, when you hand a shopkeeper a £10-00 note to cover a purchase, the value of the goods purchased + the change you receive must equal £10-00.