Question

Measurement of the absorbance of a solution containing nadh in a path length of 1cm cuvette at 340 nm shows the value of 0.31. The molar extinction coefficient of nadh is 6200 m-1 cm-1. The concentration of nadh in the solution is ___ m (correct to integer number).

Answer 1

(1) (SAMPLE EXERCISE) How many moles and how many milligrams of sodium chloride are present in 15 μl of a 0.5-M sodium chloride solution? (MNaCl = 58 g/mol)

When

1 L solution contains

0,5 mol NaCl,

then

1 mL contains

0,5mmol NaCl,

and

1 μl contains

0,5 μmol NaCl.

When 1 μl solution contains 0.5 μmol, then 15 μl contains 15 times more, which is 7.5 μmol.

This equals 7.5·10-6 mol.

The calculation can be solved by using the rule of three:

1: 0,5 = 1,5·10-5: x

x = 7,5·10-6 mol

This way, we obtain the amount of NaCl in grams by multiplying x by the molar mass. Then we convert the result to milligrams:

7.5·10-6 mol · 58 g/mol = 4.35·10-4 g = 0.435 mg