The Law of Conservation of Mass states that matter cannot be created nor destroyed. The first two reactants have a total of 23 grams so therefore the product must have 23 grams as well.
23 - 16 = 7
There would be 7 grams of hydrogen gas.
PbO
Not sure how to show crisscross method
Make sure adding the charges together=0 and multiply the elements when necessary to balance charge