Question

PBS (phosphate buffered saline) is a common salt solution used in a number of protocols including protein assays, cell work, etc. The recipe for 10X PBS is as follows:

Dissolve the following into 800 mL distilled H₂0:

80 g of NaCl (MW = 58.44 g/mol)

2.0 g of KCl (MW = 74.55 g/mol)

2.4 g of KH₂PO₄ (MW = 136.09 g/mol)

14.4 g of Na₂HPO₄ (MW = 141.96 g/mol)

Adjust pH to 7.4.

Adjust volume to 1 L with additional distilled H₂0.

Sterilize by autoclaving.

5) UH OH! You made a mistake making the 1X solution. You added 100 mL 10X stock and 1000 mL H₂0.

What X concentration of PBS did you make?

What is the concentration of Na₂HPO₄ in this dilution?

Answer 1

The recipe for 10X PBS is givenin the question, now if we add 100mL of 10X stock and 1000 mL oh H₂O, then the concentration of PBS so obtaioned is-

For 1L of solution concentration is 10X

There fore no. of moles of PBS= concentration x volume = 10X x 1= 10X

Now new concentration will be= no. of moles of PBS/ (volume of solution in L )

= (10X )/((1000+100)/1000)= (10X)/1.1= 9.09X

we can also calculate this by molarity equation as

M₁V₁=M₂V₂

M₁=10X, V₁= 1L, V₂= 1.1 L

=M₂₌(M₁V₁)/V₂= (10X x 1)/1.1= 9.09X

The concentration of the new PBS solution is 9.09X

When the concentration of PBS solution was 10X , concentration of Na₂HPO₄ was-

Molarity = (no. of moles of Na₂HPO₄)/ volume os solution in L

= (Given mass of Na₂HPO₄/molar mass of Na₂HPO₄)/volume os solution in L

=(14.4/141.96)/1= 0.1 M

By using molarity equation we can calculate the new molarity of Na₂HPO₄

M₁V₁=M₂V₂

M₁=0.1 M, V₁= 1L, V₂= 1.1 L

=M₂₌(M₁V₁)/V₂= (0.1 X 1)/1.1= 0.09 M