How many grams of Na-benzoate (powder) do I need to prepare a buffer with pH = 4.2?
How many grams of salt (NaCl) do I need to add to this buffer to make it a saturated solution of the salt?
Thanks
Benzoic acid = HBz
sodium benzoate = NaBz.
Bz^- + HOH --> HBz + OH^-
Prepare an ICE chart and substitute into the following:
Kb = (Kw/Ka) = (OH^-)^2/(Bz^-)
You know Kw and Ka. Use pH to find OH^-. Solve for (Bz^-) = molar concn of sodium benzoate. How much do you wish to prepare. M = moles/L, solve for moles, then moles = grams/molar mass and solve for grams. This will not be a buffer solution, as such, but it will have a pH of 4.2. I don't know the solubility of NaCl in such a solution nor do I have any graphs that will tell me that. The addition of NaCl will not change the pH of the solution.
To calculate the amount of Na-benzoate needed to prepare a buffer with pH 4.2, we need some additional information such as the final volume of the buffer solution and the pKa value of benzoic acid.
Assuming that you already have the necessary information, here's how you can calculate the grams of Na-benzoate to prepare the buffer:
1. Determine the desired concentration of Na-benzoate in the buffer solution. Let's say you want a concentration of 0.1 M.
2. Calculate the molar mass of Na-benzoate. The molecular formula for Na-benzoate is C7H5O2Na. The atomic masses are: C = 12.01 g/mol, H = 1.01 g/mol, O = 16.00 g/mol, and Na = 22.99 g/mol. Adding these atomic masses together gives us a molar mass of 144.11 g/mol.
3. Use the equation: mass = moles x molar mass to find the mass of Na-benzoate needed. Convert the desired concentration to moles per liter (mol/L) by multiplying it by the final volume of the buffer solution in liters.
For example, if you want to prepare 1 liter of the buffer solution:
mass = 0.1 mol/L x 1 L x 144.11 g/mol = 14.411 grams of Na-benzoate
Now, concerning the second question:
To make the buffer solution a saturated solution of NaCl, we need to find out the maximum solubility of NaCl in water at the desired temperature.
1. Determine the maximum solubility of NaCl at the temperature you're working with. This information can be found in solubility tables or online resources. Let's assume that, at the given temperature, the solubility of NaCl is 36 g/100 mL of water.
2. Calculate the mass of NaCl needed to make a saturated solution in the buffer. Multiply the maximum solubility by the final volume of the buffer solution.
For example, if you want to prepare 1 liter of the buffer solution:
mass = (36 g/100 mL) x (1000 mL/1 L) = 360 grams of NaCl
Please note that the solubility of NaCl may vary depending on temperature and other conditions. It is essential to refer to reliable sources for solubility data specific to your experimental conditions.