A buffer is prepared consisting of 0.75 mol of sodium dihydrogenphosphate, NaH2PO4, and 0.60 mol of sodium hydrogenphosphate, Na2HPO4, dissolved in 600.0 mL of water.
The Ka for the dihydrogenphosphate ion is 6.2 x 10–8 at 25°C.
Which of the following is another possible way to make the buffer described in the data set above?
a.
Add 0.6 mol of HCl to 1.35 mol of NaH2PO4 in 600.0 mL of water.
b.
Add 0.6 mol of NaOH to 1.35 mol of NaH2PO4 in 600.0 mL of water.
c.
Add 0.6 mol of HCl to 1.35 mol of Na2HPO4 in 600.0 mL of water.
d.
Add 0.6 mol of NaOH to 1.35 mol of Na2HPO4 in 600.0 mL of water.
To prepare a buffer solution, we need a weak acid and its conjugate base or a weak base and its conjugate acid.
In the given data set, we have 0.75 mol of NaH2PO4 (sodium dihydrogenphosphate) and 0.60 mol of Na2HPO4 (sodium hydrogenphosphate). NaH2PO4 is a weak acid and Na2HPO4 is its conjugate base.
To make the buffer, we need to mix these two components in a suitable ratio.
Option a: Add 0.6 mol of HCl to 1.35 mol of NaH2PO4 in 600.0 mL of water.
This option does not provide the correct components for a buffer. HCl is a strong acid, not a weak acid like NaH2PO4.
Option b: Add 0.6 mol of NaOH to 1.35 mol of NaH2PO4 in 600.0 mL of water.
This option does not provide the correct components for a buffer. NaOH is a strong base, not a weak base like Na2HPO4.
Option c: Add 0.6 mol of HCl to 1.35 mol of Na2HPO4 in 600.0 mL of water.
This option provides the correct components for a buffer. Na2HPO4 is a weak base, and adding HCl (a strong acid) converts some of the Na2HPO4 to NaH2PO4, which is the weak acid component of the buffer.
Option d: Add 0.6 mol of NaOH to 1.35 mol of Na2HPO4 in 600.0 mL of water.
This option does not provide the correct components for a buffer. NaOH is a strong base, not a weak base like Na2HPO4.
Therefore, the correct answer is option c: Add 0.6 mol of HCl to 1.35 mol of Na2HPO4 in 600.0 mL of water.
To determine if one of the given options is another possible way to make the buffer, we need to consider the components of the buffer and their concentrations.
In the given buffer, we have 0.75 mol of NaH2PO4 and 0.60 mol of Na2HPO4 dissolved in 600.0 mL of water.
Let's analyze the options:
a. Add 0.6 mol of HCl to 1.35 mol of NaH2PO4 in 600.0 mL of water.
This option suggests adding HCl to NaH2PO4. However, this would result in the formation of NaCl, not Na2HPO4. Therefore, this option is not correct.
b. Add 0.6 mol of NaOH to 1.35 mol of NaH2PO4 in 600.0 mL of water.
This option suggests adding NaOH to NaH2PO4. Again, this would result in the formation of NaCl, not Na2HPO4. Therefore, this option is not correct.
c. Add 0.6 mol of HCl to 1.35 mol of Na2HPO4 in 600.0 mL of water.
This option suggests adding HCl to Na2HPO4. This reaction would result in the formation of NaCl and H2PO4-, which is the conjugate base of H3PO4. Since we want a buffer with both the acid (H2PO4-) and the conjugate base (HPO4^2-), this option is correct.
d. Add 0.6 mol of NaOH to 1.35 mol of Na2HPO4 in 600.0 mL of water.
This option suggests adding NaOH to Na2HPO4. Again, this reaction would result in the formation of NaCl and OH-, not NaH2PO4. Therefore, this option is not correct.
Therefore, the correct answer is option c. Add 0.6 mol of HCl to 1.35 mol of Na2HPO4 in 600.0 mL of water.
The easiest way to do this is to work a-d and see if they provide the same thing. Example:
What you have.
......H2PO4^- + OH^- ==> HPO4^2-
......0.75................0.6
Or....HPO4^2- + H^+ ==> H2PO4^-
.......0.6...............0.75
Now try a,b, etc and see which gives you one of the above. Post your work you get stuck. You notice right off the bat that a can't work. H2PO4^- + H^+ makes H3PO4 and that isn't what you want.
d can't possible work because
HPO4^2- + OH^- ==> PO4^3- and that isn't what you want. So I think the answer must be b or c.