How would you calculate the pH of the buffer if 1.0mL of 5.0M NaOH is added to 20.0mL of this buffer?
Can someone please explain to me how to do B and C step by step so I could understand it clearly? :)
Say, for example, that you had prepared a buffer in which you mixed 10.30 g of sodium hydrogen phosphate, Na2HPO4, with 100.0 mL of 1.0 M sodium dihydrogen phosphate, NaH2PO4.
A. Calculate the pH of this buffer.
B. Calculate the resulting pH if 1.0 mL of 5.0 M NaOH is added to 20.0 mL of this buffer.
C. Calculate the mL of HCl that would have to be added to this buffer in order to lower the pH by exactly 1.0 unit.
Im not sure if I did A correctly, but...
Ka = 3.6x10^-13
pKa = -log(3.6x10,^-13)
= 12
10.30g Na2HPO4 x (1 mol Na2HPO4/ 141.96g Na2HPO4)
= 0.7256 mol Na2HPO4 (1.0 mol/1 L) x .1L
= .1 mol NaH2PO4
pH = 12 + log(.1 mol NaH2PO4/.07256 mol Na2HPO4)
= 12.14
For B. How exactly would I write the chemical equation and start the ICF table? And definitely need help on C. :(
Thank you soo much in advance.
I worked this for you last night.
To calculate the pH of a buffer solution, you need to consider the dissociation of the weak acid (NaH2PO4) and its conjugate base (Na2HPO4).
A. To start, you correctly determined the concentration of NaH2PO4 as 0.1 mol in 100.0 mL (or 0.1 M). Then, using the Henderson-Hasselbalch equation, you calculated the pH of the buffer as 12.14.
B. In part B, you are given that 1.0 mL of 5.0 M NaOH is added to 20.0 mL of the buffer. This means that the NaOH will react with the NaH2PO4 in the buffer to form additional Na2HPO4.
To set up the chemical equation, you can write:
NaH2PO4 + NaOH → Na2HPO4 + H2O
To calculate the resulting pH, you can follow these steps:
1. Determine the initial moles of NaH2PO4 in the buffer solution using the concentration and volume given:
Initial moles of NaH2PO4 = concentration (M) x volume (L)
2. Determine the moles of NaOH added to the solution using the concentration and volume given:
Moles of NaOH added = concentration (M) x volume (L)
3. Determine the moles of Na2HPO4 formed by the reaction using stoichiometry:
Moles of Na2HPO4 formed = moles of NaOH added (Note: The stoichiometry of the reaction tells us that the moles of NaOH added is equal to the moles of Na2HPO4 formed.)
4. Determine the final concentration of NaH2PO4 and Na2HPO4 in the solution:
Final concentration of NaH2PO4 = initial moles of NaH2PO4 (from step 1) / total volume
Final concentration of Na2HPO4 = initial moles of Na2HPO4 + moles of Na2HPO4 formed (from step 3) / total volume
5. Use the Henderson-Hasselbalch equation to calculate the resulting pH:
pH = pKa + log[concentration of NaH2PO4 / concentration of Na2HPO4]
C. In part C, you need to calculate the amount of HCl needed to lower the pH of the buffer by exactly 1.0 unit. This is done by using the Henderson-Hasselbalch equation again and adjusting the concentrations of the acid and base components in the buffer.
To solve for C, follow these steps:
1. Calculate the current concentrations of NaH2PO4 and Na2HPO4 in the buffer using the given information.
2. Use the Henderson-Hasselbalch equation to calculate the current pH of the buffer.
3. Adjust the concentrations of NaH2PO4 and Na2HPO4 in the buffer to achieve a 1.0 unit pH decrease. This can be done by increasing the concentration of the acid and decreasing the concentration of the base.
4. Calculate the new concentrations of NaH2PO4 and Na2HPO4 in the buffer after adjusting.
5. Use the Henderson-Hasselbalch equation with the adjusted concentrations to determine the new pH.
6. Determine the volume of HCl needed to achieve the desired pH decrease. This can be done using the concentration of the HCl solution and the new moles of NaH2PO4 formed.
Remember to use appropriate units and perform any necessary conversions throughout the calculations.