To create a 0.1 M carbonate buffer pH = 10.2. You choose to use a combination of HCO3- / CO32-. This buffer system has pKa = 9.9.
a) Calculate how much you need to weigh in each of the sodium salts, NaHCO3 and Na2CO3, to create 1.0 L carbonate (with total concentration of carbonate of 0.1 M).
b) In the lab you have available a 0.25M hydrogen carbonate (HCO3 - solution) and a 0.50 M carbonate (CO32 - solution). Calculate the volume you need of each of these solutions to make 1.0 L carbonate buffer (0.1 M).
To create a 0.1 M carbonate buffer with pH = 10.2 using a combination of HCO3- and CO32-, you'll need to calculate the amount of each sodium salt required to make 1.0 L of the carbonate buffer.
a) To calculate the amount of each sodium salt, you need to consider the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
In this case, A- represents CO32- and HA represents HCO3-. The pKa is given as 9.9, and the desired pH is 10.2.
1) Calculate the ratio of [A-]/[HA] using the Henderson-Hasselbalch equation.
10.2 = 9.9 + log([CO32-]/[HCO3-])
Rearranging the equation, we get:
log([CO32-]/[HCO3-]) = 10.2 - 9.9
log([CO32-]/[HCO3-]) = 0.3
Using the properties of logarithms, we can rewrite this as:
[CO32-]/[HCO3-] = 10^0.3
[CO32-]/[HCO3-] = 2
2) Since the total concentration of carbonate should be 0.1 M, we can express [HCO3-] in terms of [CO32-]:
[HCO3-] = (0.1 M) / (2 + 1)
[HCO3-] = 0.0333 M
3) Now, calculate the amount (in moles) of each solution you need to weigh. Remember that molarity (M) is moles per liter.
For NaHCO3:
Moles of NaHCO3 = [HCO3-] x Volume (in liters)
Moles of NaHCO3 = (0.0333 M) x (1.0 L)
Moles of NaHCO3 = 0.0333 moles
For Na2CO3:
Moles of Na2CO3 = [CO32-] x Volume (in liters)
Moles of Na2CO3 = (0.1 M) x (1.0 L)
Moles of Na2CO3 = 0.1 moles
4) Calculate the molecular weight of each sodium salt.
The molecular weight of NaHCO3 is 84.01 g/mol, and the molecular weight of Na2CO3 is 105.99 g/mol.
5) Calculate the weight of each sodium salt.
Weight of NaHCO3 = Moles of NaHCO3 x Molecular weight of NaHCO3
Weight of NaHCO3 = 0.0333 moles x 84.01 g/mol
Weight of NaHCO3 = 2.79 grams
Weight of Na2CO3 = Moles of Na2CO3 x Molecular weight of Na2CO3
Weight of Na2CO3 = 0.1 moles x 105.99 g/mol
Weight of Na2CO3 = 10.60 grams
Therefore, you need to weigh approximately 2.79 grams of NaHCO3 and 10.60 grams of Na2CO3 to create 1.0 L of the carbonate buffer with a total concentration of 0.1 M.
b) In this case, you already have solutions of HCO3- (0.25 M) and CO32- (0.50 M) available in the lab. So the calculations will differ.
To make 1.0 L of 0.1 M carbonate buffer, you need to determine the volume of each solution required. Let's assume we need x liters of the HCO3- solution and y liters of the CO32- solution.
1) Calculate the moles of each solution required:
Moles of HCO3- required = (0.1 M) x 1.0 L = 0.1 moles
Moles of CO32- required = (0.1 M) x 1.0 L = 0.1 moles
2) Use the molarity and volume relationship to calculate the volume of the solutions:
Volume of HCO3- solution (0.25 M) = Moles of HCO3- required / Molarity of HCO3- solution
x = 0.1 moles / 0.25 M
x = 0.4 L (or 400 mL)
Similarly,
Volume of CO32- solution (0.50 M) = Moles of CO32- required / Molarity of CO32- solution
y = 0.1 moles / 0.50 M
y = 0.2 L (or 200 mL)
Therefore, you need 400 mL of the 0.25 M HCO3- solution and 200 mL of the 0.50 M CO32- solution to make 1.0 L of the carbonate buffer with a total concentration of 0.1 M.