Volumetric

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Convert mol/given vol.(acid/base) to conc(acid/base). in mol/dm3

This method is called FIRST PRINCIPLE

Slide 42

The tips in chart

acid acid

Molar conc. Conc. in given vol. mole ratio Conc. In given vol. molar conc.

base base

Mass conc.

Mass conc.

Slide 43

Examples

20cm3 of tetraoxosulphate (vi) acid was neutralized with 25cm3 of 0.1mold-3 sodium hydroxide solution. The equation of reaction is

H2SO4 + 2NaOH Na2SO4 + 2H2O

Calculate (i) conc. of acid in moldm-3 (ii) mass conc. of the acid.

[H=1, S= 32, O=16]

Slide 44

Given:

conc. of the base = 0.1moldm-3

Vol. of the base = 25cm3

Convert to conc. in given vol.

0.1 mol in 1000cm3

X mol in 25cm3

X = 0.1 x 25

1000

0.0025mol(per25cm3)

Use mole ratio

Acid : base

1 : 2

X : 0.0025

X = 0.00125mol(in given vol of the acid) i.e 20cm3

Convert to conc.(acid) in moldm-3

0.00125mol in 20cm3

X in 1000cm3

X = 0.0625mol.

.: conc. of acid

= 0.0625moldm-3

Slide 45

Example 1 continues

ii mass conc. of the acid :

Mass conc. = molar conc. X molar mass

0.0625 x [2+32+64]

0.0625 x 98=6.13gdm-3

Remember, always leave your answers in 3 s.f.

Slide 46

More

If 18.50cm3 hydrochloric acid were neutralized by 25cm3 of potassium hydroxide solution containing 7gdm-3. what is the conc. of the acid in moldm-3?

The equation of reaction:

HCl + KOH HCl + H2O

[K = 39, O = 16, H = 1]

Slide 47

Let’s solve it together

Given:

Mass conc. of the base = 7gdm-3

Convert to moldm-3 : Mass conc. = 7

Molar mass [39+16+1]

= 0.125 moldm-3

Mol reacted at the given vol.(25cm3)

n = conc. in moldm-3 x vol.(dm3) 0.125 x 25/1000

0.003125mol

Using mole ratio

Acid : base

1 : 1

X : 0.003125

X = 0.003125

0.003125mol[per18.5cm3] in moldm-3

0.003125mol in 18.5cm3

X in 1000cm3 x = 0.169mol

.: conc. of the acid

= 0.169 moldm-3

Slide 48

You can use “the theory”

CaVa = na

CbVb nb

Example 1 again.

20cm3 of tetraoxosulphate (vi) acid was neutralized with 25cm3 of 0.1mold-3 sodium hydroxide solution. The equation of reaction is

H2SO4 + 2NaOH Na2SO4 + 2H2O

Calculate (i) conc. of acid in moldm-3 (ii) mass conc. of the acid.