Why are the atomic masses on the periodic table not listed as whole numbers?

Iknow the answer to 13 is b and 14 is d. i just need to know why the correct answers are correct

7. what temperature is need to just dissolve 50 g of nh4cl in 75 g of water? '

Select the correct answer. when carbon dioxide dissolves in water, it sometimes reacts with water to form carbonic acid as in this balanced equation: co2 + h2o → h2co3. if 495 milliliters of carbon dioxide at 25°c and 101.3 kilopascals reacts with excess water, what is the theoretical yield of carbonic acid? use the periodic table and the ideal gas resource a. 0.889 g b. 1.10g c. 1.27g d. 2.02g what's the answer! quick!