Most of these look like dilution questions which follow a general pattern;
Use M1V1 = M2V2 (because moles in a solution stays constant if diluted)
so, the Concentration before * Volume before = Concentration after * Volume after
Using the first question as an example; 0.2*50 = 100 * M2
Rearranging for M2: 0.1
Therefore the concentration of the final solution is 0.1 M
we know that to work out pH you must use negative logs to base 10; i.e. -log(0.1) gives you the pH for this solution which is 1
