a) Calculate how many cm3 of 1.00 moldm-3 HCL are needed to decrease the pH of 1.00 dm3 of HCL by one unit. Ignore the small change in volume
b) Calculate how many cm3 1.00moldm-3 NaOH are needed to increase the pH of 1.00dm3 of HCL by one unit. Ignore the small change in volume
b. concentration of 1.00 mol dm-3 NaOH = 1mmol cm-3 ; so 0.9 cm3 has 0.9mmol.
Don't you have a starting pH of the 1 dm3? At a pH of anything smaller than pH = 3 the volume added can't be ignored. If you assume pH = 3 and you want it to be 1 unit less or pH = 2 then you want the make the final concn 10 times greater. I would look at it this way.
You have pH = 3 or (H^+) = (HCl) = 0.001 so you have 1000 mL x 0.001 = 1 millimole HCl. You want 10 millimoles (since 10 mmol/1000 mL - 0.01) so you will need to add 9 millimoles to do that. You can get that from
9 mL of 1 M HCl (that's 1 mol/dm3
The NaOH is done the same way. Post your work if you get stuck.
To calculate how many cm3 of 1.00 moldm-3 HCl are needed to decrease the pH of 1.00 dm3 of HCl by one unit, you need to know the initial pH of the solution.
Once you have the initial pH value, you can use the equation:
[H+] = 10^(-pH)
This equation allows you to calculate the concentration of H+ ions in the solution. Since the question asks for 1.00 moldm-3 HCl, we know that the concentration of H+ ions is equal to the concentration of HCl.
To decrease the pH by one unit, the new pH will be the initial pH minus 1. The new concentration of H+ ions is then calculated using the new pH value as:
[H+] = 10^(-new pH)
To find the volume of HCl needed, divide the difference in concentration by the initial concentration and multiply by the volume of the solution:
Volume = (Initial concentration - New concentration) * Volume of solution
For the second part of the question, "Calculate how many cm3 of 1.00 moldm-3 NaOH are needed to increase the pH of 1.00 dm3 of HCl by one unit," the process is similar, but we need to consider that NaOH is a base and reacts with HCl in a 1:1 ratio.
First, calculate the concentration of OH- ions that will be needed to increase the pH by 1 unit:
[OH-] = 10^(-pOH)
Since NaOH is a strong base, its concentration is the same as the concentration of OH- ions. Now, calculate the volume of NaOH needed using the same formula as before:
Volume = (New concentration - Initial concentration) * Volume of solution
Make sure to note the units you are using, as the question asks for cm3 and dm3.