You are instructed to create 200. mL of a 0.27 M phosphate buffer with a pH of 7.8. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.)
H3PO4(s) + H2O(l) equilibrium reaction arrow H3O+(aq) + H2PO4−(aq)
Ka1 = 6.9 ✕ 10−3
H2PO4−(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + HPO42−(aq)
Ka2 = 6.2 ✕ 10−8
HPO42−(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + PO43−(aq)
Ka3 = 4.8 ✕ 10−13
Which of the available chemicals will you use for the acid component of your buffer?
H3PO4
NaH2PO4
Na2HPO4
Na3PO4
Which of the available chemicals will you use for the base component of your buffer?
H3PO4
NaH2PO4
Na2HPO4
Na3PO4
What is the molarity needed for the acid component of the buffer?
What is the molarity needed for the base component of the buffer?
How many moles of acid are needed for the buffer?
How many moles of base are needed for the buffer?
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mol
How many grams of acid are needed for the buffer?
How many grams of base are needed for the buffer?
To create a phosphate buffer with a pH of 7.8, we need to use a combination of an acid and its conjugate base.
To determine the acid component of the buffer, we look for the chemical that will act as the acid in the buffer solution. In this case, the acid component will be the chemical that donates a proton (H+) to the solution. Looking at the available chemicals, H3PO4 is the only one that can donate a proton, so we will use H3PO4 as the acid component of the buffer.
To determine the base component of the buffer, we look for the chemical that will act as the base in the buffer solution. The base component will be the chemical that accepts a proton (H+) from the solution. Looking at the available chemicals, Na2HPO4 and Na3PO4 can accept a proton and act as a base in the buffer solution. However, Na3PO4 is a stronger base compared to Na2HPO4, so we will use Na2HPO4 as the base component of the buffer.
Next, we need to determine the molarity (concentration) needed for the acid component of the buffer. The molarity of the acid component is equal to the desired pH value. In this case, the desired pH is 7.8. Therefore, the molarity of the acid component is 7.8 M.
Similarly, we need to determine the molarity needed for the base component of the buffer. The molarity of the base component can be calculated using the formula: Molarity = 10^(-pOH). Since pH + pOH = 14 (at 25°C), we can calculate the pOH from the desired pH. In this case, the desired pH is 7.8, so the pOH is 14 - 7.8 = 6.2. Therefore, the molarity of the base component is 10^(-6.2) M.
To determine the number of moles of acid and base needed for the buffer, we need to multiply the molarity by the volume. Given that the volume of the buffer is 200 mL (or 0.2 L), we can calculate the moles of acid and base using the formula: Moles = Molarity x Volume.
The number of moles of acid needed for the buffer is calculated as follows:
Moles of acid = Molarity of acid x Volume of buffer
= 7.8 M x 0.2 L
= 1.56 moles
The number of moles of base needed for the buffer is calculated as follows:
Moles of base = Molarity of base x Volume of buffer
= (10^(-6.2) M) x 0.2 L
= 0.000158 moles
To calculate the mass of acid and base needed for the buffer, you need to know the molar mass of each chemical. The molar mass of H3PO4 is 97.994 g/mol, and the molar mass of Na2HPO4 is 141.96 g/mol.
To calculate the mass of acid:
Mass of acid = Moles of acid x Molar mass of acid
= 1.56 moles x 97.994 g/mol
= 153.07 g
To calculate the mass of base:
Mass of base = Moles of base x Molar mass of base
= 0.000158 moles x 141.96 g/mol
= 0.0224 g
Therefore, you will need 1.56 moles (153.07 g) of H3PO4 and 0.000158 moles (0.0224 g) of Na2HPO4 to create the desired phosphate buffer solution.