How much 1M HCl should I add to 100 mL H2O to lower its pH by 1?

Question

It's for a Biology project- I need to have different pH of solutions, with my base point being water with a pH of 7, and then increments down to a pH of 3. I don't know how much HCl to add for each increment (increments of pH of 3, 4, 5, 6, 7)

Answer 1

To get a pH of 3, the [H+] needs to be 10^-3 M.  For pH 4, it needs to be 10^-4 M, etc.

So, since you are starting with 1 M HCl = 1 M [H+], you can make simple dilutions from that.

 

Of course, the amount of HCl you use will depend on what final volume you desire.  Let's assume you want to make 100 mls of pH 3, 4, 5, and 6.

 

For pH 3:  (x ml)(1 M) = (100 ml)(1x10^-3 M)

   x = 0.1 ml.  So take 0.1 ml of 1 M HCl and dilute to a final volume of 100 mls

 

For pH 4:  (x ml)(1 M) = (100 ml)(1x10^-4 m)

   x = 0.01 ml.  So take 0.01 ml (10 ul) and dilute to a final volume of 100 mls

 

For pH 5:

   same calculation and x = 0.001 mls.  This is too small to really measure, so you can take some of the previously made pH 4 solution and dilute that 1:10.  Take 10 mls of pH4 solution and dilute to a final volume of 100 mls.

 

For pH 6:  

   Take 10 mls of pH 5 solution and dilute to 100 mls final volume 

 

NOTE:  You can start with making the pH3 and then just keep making 1:10 dilutions from that. In other words, make the pH 3 as described, and then 10 mls diluted to 100 mls gives you pH4.  Then dilute that 1:10 to get pH, and dilute that to get pH 6.  Water will be your pH 7 (assuming it is pure and actually at pH 7).