So the main concept here is buffers. You need to understand what a buffer is. A buffer is simply a compound that has both acidic and basic properties so when you add in another solution such as acid or base, it needs to first neutralize the opposing base (if adding acid) or acid (if adding in base) before it starts to drastically affect the pH. So when you talk about phosphate buffers, you are referring to buffers that contain the PO4(2-) group. Now, it says that it has 3 pKa values. These are known as the equivalence points, or pH at which the acid and base are equal (hence making a great buffer). H3PO4 --> H+ + H2PO4(-1) As you as can see, there are 3 hydrogens making it extremely acidic thereby it is the one that has the lowest pKa (2.2) H2PO4 (-1) --> H+ + HPO4(-2) This has the pKa of 7.2 and the last step is the hardest to remove hydrogen and has the pKa of 12.7 Now if you want a phosphate buffer at 7.4, you can\'t only use the H2PO4 which has a pKa of 7.2. So to compensate, you need some of the HPO4 (-2) degradation with a pKa of 12.7 to balance it out. It is really simple math once you figure out what they are asking for. Solution So the main concept here is buffers. You need to understand what a buffer is. A buffer is simply a compound that has both acidic and basic properties so when you add in another solution such as acid or base, it needs to first neutralize the opposing base (if adding acid) or acid (if adding in base) before it starts to drastically affect the pH. So when you talk about phosphate buffers, you are referring to buffers that contain the PO4(2-) group. Now, it says that it has 3 pKa values. These are known as the equivalence points, or pH at which the acid and base are equal (hence making a great buffer). H3PO4 --> H+ + H2PO4(-1) As you as can see, there are 3 hydrogens making it extremely acidic thereby it is the one that has the lowest pKa (2.2) H2PO4 (-1) --> H+ + HPO4(-2) This has the pKa of 7.2 and the last step is the hardest to remove hydrogen and has the pKa of 12.7 Now if you want a phosphate buffer at 7.4, you can\'t only use the H2PO4 which has a pKa of 7.2. So to compensate, you need some of the HPO4 (-2) degradation with a pKa of 12.7 to balance it out. It is really simple math once you figure out what they are asking for..

1. So the main concept here is buffers. You need to understand what a buffer is. A
buffer is simply a compound that has both acidic and basic properties so when you add in
another solution such as acid or base, it needs to first neutralize the opposing base (if adding
acid) or acid (if adding in base) before it starts to drastically affect the pH. So when you talk
about phosphate buffers, you are referring to buffers that contain the PO4(2-) group. Now, it
says that it has 3 pKa values. These are known as the equivalence points, or pH at which the acid
and base are equal (hence making a great buffer). H3PO4 --> H+ + H2PO4(-1) As you as can
see, there are 3 hydrogens making it extremely acidic thereby it is the one that has the lowest
pKa (2.2) H2PO4 (-1) --> H+ + HPO4(-2) This has the pKa of 7.2 and the last step is the
hardest to remove hydrogen and has the pKa of 12.7 Now if you want a phosphate buffer at 7.4,
you can't only use the H2PO4 which has a pKa of 7.2. So to compensate, you need some of the
HPO4 (-2) degradation with a pKa of 12.7 to balance it out. It is really simple math once you
figure out what they are asking for.
Solution
So the main concept here is buffers. You need to understand what a buffer is. A
buffer is simply a compound that has both acidic and basic properties so when you add in
another solution such as acid or base, it needs to first neutralize the opposing base (if adding
acid) or acid (if adding in base) before it starts to drastically affect the pH. So when you talk
about phosphate buffers, you are referring to buffers that contain the PO4(2-) group. Now, it
says that it has 3 pKa values. These are known as the equivalence points, or pH at which the acid
and base are equal (hence making a great buffer). H3PO4 --> H+ + H2PO4(-1) As you as can
see, there are 3 hydrogens making it extremely acidic thereby it is the one that has the lowest
pKa (2.2) H2PO4 (-1) --> H+ + HPO4(-2) This has the pKa of 7.2 and the last step is the
hardest to remove hydrogen and has the pKa of 12.7 Now if you want a phosphate buffer at 7.4,
you can't only use the H2PO4 which has a pKa of 7.2. So to compensate, you need some of the
HPO4 (-2) degradation with a pKa of 12.7 to balance it out. It is really simple math once you
figure out what they are asking for.