8-101 Suppose you have an aqueous solution prepared by dissolving 0.050 mol of NaH2PO4 in 1 L of water. This solution is not a buffer, but suppose you want to make it into one. How many moles of solid Na2HPO4 must you add to this aqueous solution to make it into:
(a) A buffer of pH 7.21
(b) A buffer of pH 6.21
(c) A buffer of pH 8.21
(a)
Interpretation:
The number of moles to be added in NaH2 PO4 to make it into a buffer of pH 6.21 should be calculated.
Concept Introduction:
The pH of buffer solution is calculated using the following formula:
This is known as Henderson- Hasselbalch equation. Here,
Answer to Problem 8.101P
Explanation of Solution
Given Information:
An aqueous solution contains 0.050 moles of NaH2 PO4.
The volume of the solution is 1 L.
The pH of the buffer solution is 7.21.
A buffer is a solution which resists change in pH when limited amounts of an acid or a base are added to it. The given buffer is made of the weak acid, NaH2 PO4 and its conjugate base Na2 HPO4.
The Henderson-Hasselbalch equation for a buffer of a weak acid (HA) and its conjugate base(A- ) is given as:
By using the Henderson-Hasselbalch equation for the pH of the buffer solution we get,
As, we know that pKa of NaH2 PO4 is 7.21.
The volume of the buffer solution is 1 L. Assuming that there will not be any change in the volume of solution due to the addition of Na2 HPO4, the concentration terms in the Henderson-Hasselbalch equation can be replaced by the number of moles.
By substituting the values of pH, pKa and the number of moles of NaH2 PO4 for [NaH2 PO4 ] in the above equation, we get.
Bu taking antilog on both sides we get,
Thus,
(b)
Interpretation:
The number of moles to be added in NaH2 PO4 to make it into a buffer of pH 7.21 should be calculated.
Concept Introduction:
The pH of buffer solution is calculated using the following formula:
This is known as Henderson- Hasselbalch equation. Here,
Answer to Problem 8.101P
Explanation of Solution
Given Information:
An aqueous solution contains 0.050 moles of NaH2 PO4.
The volume of the solution is 1 L.
The pH of the buffer solution is 6.21.
By using the Henderson-Hasselbalch equation for the pH of the buffer solution we get,
As, we know that pKa of NaH2 PO4 is 7.21.
The volume of the buffer solution is 1 L. Assuming that there will not be any change in the volume of solution due to the addition of Na2 HPO4, the concentration terms in the Henderson-Hasselbalch equation can be replaced by the number of moles.
By substituting the values of pH, pKa and the number of moles of NaH2 PO4 for [NaH2 PO4 ] in the above equation, we get.
Bu taking antilog on both sides we get,
Thus,
(c)
Interpretation:
The number of moles to be added in NaH2 PO4 to make it into a buffer of pH 8.21 should be calculated.
Concept Introduction:
The pH of buffer solution is calculated using the following formula:
This is known as Henderson- Hasselbalch equation. Here,
Answer to Problem 8.101P
Explanation of Solution
Given Information:
An aqueous solution contains 0.050 moles of NaH2 PO4.
The volume of the solution is 1 L.
The pH of the buffer solution is 8.21.
By using the Henderson-Hasselbalch equation for the pH of the buffer solution we get,
As, we know that pKa of NaH2 PO4 is 7.21.
The volume of the buffer solution is 1 L. Assuming that there will not be any change in the volume of solution due to the addition of Na2 HPO4, the concentration terms in the Henderson-Hasselbalch equation can be replaced by the number of moles.
By substituting the values of pH, pKa and the number of moles of NaH2 PO4 for [NaH2 PO4 ] in the above equation, we get.
Bu taking antilog on both sides we get,
Thus,
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Introduction to General, Organic and Biochemistry