What action is the best example of a direct observation

If the mass of the products measured 120g what would the mass of the reactants a. 30g b. 60g c. 120g d. 240g

What is the relation between concentration of reactants and the rate of chemical reaction?

In 1901, thomas edison invented the nickel-iron battery. the following reaction takes place in the battery. fe(s) + 2 nio(oh)(s) + 2 h2o(l) fe(oh)2(s) + 2 ni(oh)2(aq) how many mole of fe(oh)2, is produced when 5.35 mol fe and 7.65 mol nio(oh) react?