148.FUNDAMENTALS OF ANALYTICAL CHEMISTRY(12) – Concentration units(7).

1. Calculate gram equivalents of the following – (Given – Eq.wt of O=8,Al =9)
i)7 × 10-3 kg of oxygen.
ii) 4.5 × 10-3 kg of aluminium

A: Formula used – Gram equivalent =w/EW.
Note that the weight of the elements are given in kg. So, we need to first convert them to grams.

1kg = 1000g i.e 103 g
So, 1g = 10-3 kg

I)Thus, 7 × 10-3 kg = 7g of oxygen.

Gram equivalent = weight of the element (w) /equivalent weight of the element(EW)
∴ Gram equivalent of oxygen – 7/8 = 0.875.

ii) Similarly, gram equivalent of aluminium = 4.5/9 = 0.5.

2.Calculate the equivalent weight of C6H5COOH. 

A: Molecular weight of C6H5COOH =  6(12)+5(1)+12+2(16)+1 = 122g .

This compound has only one replaceable hydrogen. So, n-factor =1.

Equivalent weight = Molecular weight /nf = 122 /1 = 122.

3.What is the molarity of 0.3181 N HCl?

A: We know that , Normality = molarity /nf.
∴Molarity = nf × normality.For HCL the n-factor is 1.
∴ The molarity of this acid is equal to its normality i.e 0.3181.

4.A solution has a concentration of 0.5 mg mL-1.What is its concentration in ppm?

A: ppm means mg of solute per litre of solution.The quantity given is per mL of solution.So, we have to convert ml into litres.

1000ml = 1litre
∴1ml = 10-3 L

Thus, concentration in ppm = 0.5 ÷10-3 mg L-1 = 500ppm.

5.A concentrated solution of aqueous ammonia is 28.0% w/w NH3 and has a
density of 0.899 g/mL. What is the molar concentration of NH3 in this solution?

A: %w/w means gms of solute per 100 g of solution.This means the solution of ammonia has 28g of NH3 in 100g of solution.

We know density of the solution.

Density = mass of the solution/volume.
∴Volume = mass/density = (100g)/(0.899 g/mL) = 111.2mL
Volume of solution =111.2mL= 0.1112 L.

Thus, 28g of NH3 is present in 0.11L of solution.So, in 1 liter we will have 28/0.11 = 254.5g of NH3.

no.of moles of solute = weight in 1 litre of solution/M.wt = 254.5/17=14.97 g/mole.

Thus, the molar concentration of this solution is 14.97g/mol.

We will discuss some other fundamental concepts of analytical chemistry in the next post. Till then ,

Be a perpetual student of life and keep learning…

Good day !

References and further reading –




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