How to do serial dilution problems?

The way to do serial dilutions is to take a fraction of your solution and add it to water to dilute the solute further. Since you start with 10mL of 12% salt, .04/.12= 1/3 meaning that you need to reduce your salt content down to 1/3 it's current content. If you add 2X water of the amount of your solution, you will get 1/3 the concentration of your solute (1 part solution plus 2 parts water equals parts total, 1/3 of which is your salt solution so it will cut your salt solution by 1/3). So by adding 3.33 mL of your solution to a beaker, then adding 6.67 mL water, you will get 10mL of 4% salt. Then if you add 5 mL of your 4% salt solution to a new beaker, and 5 mL water, you get one half of your salt solution or 2% salt solution. By doing it in multiple steps, you can yield small amounts of your needed solution (saving reagents) and retain accuracy. (if you started out trying to measure 4% and 2% salt solutions, you would likely measure inaccurate amounts of salt since scales have trouble distinguishing small amounts of weight from another. By starting out with a higher concentration and then measuring out fractions of it, you can get small dilutions with greater accuracy.

Now try the second one on your own, remember, just put your desired fraction over your current dilution to get the fraction you need to reduce to, then times the total desired volume of your wanted solution (you can always pick 10 mL for clarity) by the fraction to get the amount of your starting solution you should add, then add water to volume up to desired amount (10 mL minus the fraction of starting solution or 10-3.33=6.67 in the earlier example) Now try the second one on your own.