How would I make 100mL of a .100 M phosphate buffer with a pH of 7.2 using 1 M NaH2PO4 and Na2HPO4 solid (MW= 142 g/mol)?
pH = pK2 + log (base/acid)
I looked up pK2 and found 7.2; therefore,
7.2 = 7.2 + log (base/acid)
so (base/acid) = 1
If we used 100 mL of the 1 M NaH2PO4, that is 1 M for the concn of acid.
If we used 0.1 mole Na2HPO4 in 100 mL that is 0.1 mole/0.1 L = 1 M so the ratio is 1.
0.1 mole Na2HPO4 must be 14.2 grams.
Will that do it? We can see.
pH = 7.2 + log (base/acid)
pH = 7.2 + log (1 M base/1 M acid)
pH = 7.2 + 0 = 7.2
Technically this will make slightly more than 100 mL because 100 mL solution + 14.2 g of the salt will likely increase the volume slightly.
To make a phosphate buffer with a desired pH and concentration, you first need to calculate the amount of each acid and base component required. Here's the step-by-step process to make 100 mL of a 0.100 M phosphate buffer solution with a pH of 7.2 using 1 M NaH2PO4 and Na2HPO4 solids:
1. Determine the target pH: In this case, the target pH is 7.2.
2. Decide on the ratio of NaH2PO4 to Na2HPO4: A phosphate buffer consists of both the acidic form (H2PO4-) and the basic form (HPO42-) of phosphate ions. To successfully buffer at a specific pH, the ratio of each component must be correct. The Henderson-Hasselbalch equation can be used to determine the ratio:
pH = pKa + log ([A-]/[HA])
For a pH of 7.2, the pKa for the phosphate buffer is approximately 7.2. By rearranging the equation, we can determine the ratio of [A-] to [HA] required:
[A-]/[HA] = 10^(pH - pKa)
Substituting the pH and pKa values:
[A-]/[HA] = 10^(7.2 - 7.2) = 1
Therefore, a 1:1 ratio of NaH2PO4 to Na2HPO4 is needed to achieve a pH of 7.2.
3. Calculate the amount of NaH2PO4: The desired concentration of the buffer is 0.100 M. Since you need equal amounts of NaH2PO4 and Na2HPO4, each component should be 0.050 M.
To find the moles of NaH2PO4 required, use the formula:
Moles = Molarity × Volume (in liters)
Moles of NaH2PO4 = 0.050 M × 0.100 L = 0.005 mol
Next, calculate the mass of NaH2PO4 using its molar mass:
Mass = Moles × Molar Mass
Mass of NaH2PO4 = 0.005 mol × 142 g/mol = 0.71 g
You would need 0.71 grams of NaH2PO4.
4. Calculate the amount of Na2HPO4: Since you need equal amounts of Na2HPO4 as NaH2PO4, the required mass of Na2HPO4 is also 0.71 grams.
5. Dissolve the solids: Weigh out 0.71 grams of NaH2PO4 and 0.71 grams of Na2HPO4 separately. Then, dissolve each solid in distilled water and combine them in a 100 mL container. Ensure that the volume of the final solution is indeed 100 mL.
By following these steps, you can prepare a 100 mL phosphate buffer solution with a pH of 7.2 and a concentration of 0.100 M using 1 M NaH2PO4 and Na2HPO4 solids.