Astudent heats 3.55 grams of copper and 4.60 grams of iodine for 6.25 grams of a new compound. all of the iodine reacted, but there is still some copper left over unused. how much copper is left?

This item can be answered by using the concept of material balance. The amount of substance used for the reaction should be the same before and after the reaction occurred. 

Amount of reactants = 3.55 grams + 4.60 grams = 8.15 grams

We are also given that the amount of product accounted for is only 6.25 grams, this entails that not all copper was reacted. Subtracting this value from 8.15 g will give us an answer of 1.9 grams. 

1.9 g of copper

So what you would do is go through and add 3 teaspoons(tsp) 6 times because of the 6 tablespoons(tbsp). which will give you an answer of 18. another way is to multiply 3x6=18.