Which tool or tools would you use to measure and compare the mass of a cup of fresh water and a cup of salt water

You could use a scale to measure the mass as well as a cup to hold the water. If you were comparing the two, you should also probably use a graduated cylinder to get the same amount of each type of water.

Hope this helped ^_^

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2