I need help on calcualtions for a chemistry lab. These were the directions: Buff

Question

I need help on calcualtions for a chemistry lab.

These were the directions:

Buffer Calculations Practice (how much to mix?)

PART A: Make a 300 ml solution of 100mM Tris base in H2O. How do I CALCUALTE this?

Use the henderson hasselbach to make a 50 ml 50mM Tris solution at pH 8.0 by mixing Tris HCl and Tris base powders ( use formula weights of Tris-HCl and Tris base). CALCULATE How many grams of Tris HCl and Tris base did you add to make your solution?

PART B: HOW DO YOU MAKE THE FOLLOWING BUFFER; ---> What should I CALCULATE here?

Buffer: 10X TAE

0.4M Tris base, 0.01M disodium EDTA, 0.2M acetic acid

Make 500ml of this solution from chemical stocks. The pH of this solution should be between 8.2 and 8.3. THEN Make a 100 ml 1X dilution of TAE from this 10X stock. CALCULATE

*this was all the information that was given.

Explanation / Answer

PART A:

Volume of base solution = 300 mL = 0.30 L

Molarity of solution = 100 mM = 0.10 M

Moles of base = 0.30 * 0.10

= 0.030 moles

Molecular weight of Tris base = 121.14 g/mol

Mass of Tris base required = 0.030 * 121.14

= 3.63 g

Volume of buffer solution = 50 mL = 0.05 L

Molarity of solution = 50 mM = 0.050

Moles = 0.050 * 0.050

= 0.0025

acid + base = 0.0025

pH = 8.0

pKa of tris = 8.07

Using henderson hasselbach equation:

pH = pKa + log (acid / base)

8.0 = 8.07 + log (acid / base)

log (acid / base) = - 0.07

acid / base = 0.851

(0.0025 - base) / base = 0.851

Moles of base = 0.00135

Moles of acid = 0.0025 - 0.00135

= 0.00115

Mass of Tris base = 0.00135 * 121.14

= 0.163 g

Formula Weight of Tris HCl = 157.56 g/mole

Mass of Tris HCl = 0.00115 * 157.56

= 0.181 g

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