What volume of 85% H3PO4(d=1.68g/mL) and weight of KH2PO4 are required to prepare a buffer of pH=3 that has an ionic strength of 0.2?
To determine the volume of 85% H3PO4 (d=1.68 g/mL) and weight of KH2PO4 required to prepare a buffer of pH=3 with an ionic strength of 0.2, we need to follow these steps:
Step 1: Calculate the ionic strength (I) of the buffer solution.
The ionic strength (I) is calculated using the equation:
I = 0.5 * ∑(Ci * Zi^2)
Where:
- Ci represents the molar concentration of each ion in the solution,
- Zi represents the charge of the ion.
In this case, we have to consider the dissociation of both H3PO4 and KH2PO4:
H3PO4 dissociates into H+ and H2PO4-, with charges +1 and -1, respectively.
KH2PO4 dissociates into K+, H2PO4-, and PO4^3-, with charges +1, -1, and -2, respectively.
Step 2: Calculate the molar concentration of each ion.
To calculate the molar concentrations, we need the initial concentrations of the solution. Let's assume we want to prepare a 1 L buffer solution.
For H3PO4:
The density of the 85% H3PO4 solution is given as 1.68 g/mL. We can convert this to g/L by multiplying by 1000:
1.68 g/mL * 1000 mL/L = 1680 g/L.
To determine the molar concentration, we need the molar mass of H3PO4 (97.994 g/mol):
Molar concentration = mass / molar mass = 1680 g/L / 97.994 g/mol.
For KH2PO4:
We need the molecular weight of KH2PO4 (136.086 g/mol) and the mass of KH2PO4 required for the desired buffer concentration, which is not provided in the question.
Step 3: Solve for the molar concentration of each ion.
Using the molar concentrations obtained from Step 2, we can solve for the molar concentrations of each ion.
Step 4: Use the Henderson-Hasselbalch equation to solve for the ratio of acid to salt.
The Henderson-Hasselbalch equation is given by:
pH = pKa + log ([salt] / [acid])
In this case, the desired pH is 3, and we know that the pKa for H3PO4 is 2.15.
Step 5: Determine the volume of 85% H3PO4 and the weight of KH2PO4 required.
We can use the molar concentrations obtained in Step 3 and the Henderson-Hasselbalch equation from Step 4 to find the volume of H3PO4 and the weight of KH2PO4 needed to prepare the buffer solution.
Please provide the mass of KH2PO4 required, and I can help you complete the calculation.