Isotopes. They conatin atoms of the same element with the same proton number (atomic no) but different number of neutrons (atomic mass = protons + neutrons)
Try 0.80 grams NaOH . M=mol/L if you rework it to get mol, you have mol=0.20M×0.100L which is 0.02mol of NaOH, multiply that by the amu of NaOH (39.998) and should get 0.799g
its is important because without cells we wouldn't know about reproduction and they are different types of cells such as plant cells egg cells sperm cells ect without them we wouldn't know about how we are born