1. How much NaCl do you weigh out to make 100 mL of a 0.15% aqueous solution?

    0.15 grams

  2. Suppose you have a compound that has a molecular weight of 100. How much do you weigh out to make 100 mL of a 2% solution?

    2.0 grams (molec. wt has nothing to do with % concentrations)

  3. Suppose you have a compound that has a molecular weight of 100. How much of it do you weigh out to make 1 liter of a 1 molar?

    100 grams (making molar solutions is where molec. weight counts)

  4. See the containers before you. How much of which do you use to make 200 mL of a 2.5% solution of lactose?

    First you noticed that there was no solid lactose to weigh out.
    Easier way: dilute the 5% in half: 50 mL of 5% + 50 mL of water
    Harder way: dilute the 4% to 2.5%: CV=CV; 4%xV=2.5%x100 mL; V=62.5 mL of 4%.

  5. What % concentration would you get if you mixed 200 mL of 4% lactose and 200 mL of 5% lactose?

    When you mix two things, you get something in between. Since you are mixing equal quantitites, you get something exactly in the middle; thus halfway between 4 and 5 is 4.5%

    CV=CV + CV; Cx400 mL = 4x200 mL + 5x200 mL
    Cx400 = 800 + 1000 = 1800.
    1800/400 = 4.5%

  6. You need 200 mL of 0.2 molar NaCl. And so you made these calculations: 58.44 x 0.2 x (200/1000) = 2.34 grams
    What 5 concentration is that?

    2.34 grams/200 mL = 1.17 grams/100 mL; 1.17%