Amount of starting material needed for Buffer

[thephoenician]

given .1M of formic and .1M formate, how do you make 40 ml of .1M Buffer ph=2.9


basically it's going to equal .141=$$\frac{base}{acid}$$ after going through henderson hasselbach

Does anyone know how to do this using an ICE table?

 

[Borek]

ICE table is of no use here.

You have two equations. Total volume:

$$V_{base} + V_{acid} = 40$$

and ratio of concentrations (as given by Henderson-Hasselbalch equation):

$$\frac {0.1*V_{base}} {V_{base} + V_{acid}} = 0.141 * \frac {0.1*V_{acid}} {V_{base} + V_{acid}}$$

Note that what I have put on both sides is just amount of substance divided by total volume, nothing fancy. Just solve.



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[Blueshift5]

I can provide a response to this question. To make 40 ml of a .1M buffer with a pH of 2.9, you will need to use an ICE table to calculate the amount of starting material needed. First, write out the chemical equation for the buffer reaction, which in this case is formic acid (HCOOH) and formate (HCOO-).

HCOOH + H2O ⇌ H3O+ + HCOO-

Next, create an ICE table with initial, change, and equilibrium values. The initial values will be 0.1 M for both formic acid and formate. The change values will be -x for formic acid and +x for formate, since they will both decrease by the same amount as they react. The equilibrium values will be 0.1-x M for formic acid and x M for formate.

| Chemical | Initial | Change | Equilibrium |
|----------|---------|--------|-------------|
| HCOOH | 0.1 M | -x | 0.1-x M |
| HCOO- | 0.1 M | +x | x M |

Next, use the Henderson-Hasselbalch equation to calculate the pH of the buffer at equilibrium:

pH = pKa + log(\frac{[base]}{[acid]})

Since we want a pH of 2.9, we can rearrange the equation to solve for x:

x = [base] = [acid] \times 10^{pH-pKa}

Plug in the values for pH (2.9) and pKa (3.75 for formic acid) and solve for x. This will give you the equilibrium concentrations of formic acid and formate in the buffer. Then, use the equation M = \frac{moles}{volume} to calculate the moles of each compound needed for 40 ml of a 0.1 M buffer. Finally, use the molecular weight of each compound to convert moles to grams and measure out the appropriate amounts of formic acid and formate to make your buffer.