Dimensional ysis help? chemistry equations? hurry! i'm confused :p?

What you need to understand is how chemists express weight. They use a number called a mole. This many atoms of any element equals its atomic weight in the periodic table. To solve the problem, express the weight in moles. 7.5 grams / (58.44 grams mole) = 0.1283 moles NaCl. Next, refer to the chemical equation describing the reaction. It needs to be balanced, meaning there must be equal numbers of atoms on each side. Note the ratio of NaCl to Cl2 is 2 to 1. Since moles are just numbers, the same applies to the amounts as well. 0.1283 moles of NaCl will produce half this many moles of Cl2. (0.5)*(0.1283 moles) = 0.064 moles Cl2. To calculate the weight, just multiply this by the molecular weight. (0.064 moles)*(70.9 grams/mole) = 4.55 grams Cl2.