# Overview:

• Sodium hydroxide is a clear colourless liquid. A 50% (w/w) concentrated Sodium hydroxide can be obtained from different suppliers.
• 50% (w/w) Sodium hydroxide means that 100 grams of solution contains 50 grams of NaOH.
• The density of 50% (w/w) Sodium hydroxide is 1.515 g/ml at 25°C that means that weight of the 1 ml of Sodium hydroxide solution is 1.22 gram at 25°C.
• Molarity refers to number of moles of the solute present in 1 litre of solution.
• In simple words, 1 mole is equal to atomic weight of the substance. For example, 1 mole of Sodium hydroxide is equal to 40.00 grams of Sodium hydroxide (NaOH, molecular weight = 40.00).

# Calculation:

Known values:
Density of Sodium Hydroxide :                                1.515 g/ml
Molecular weight of Sodium Hydroxide:                40.00 g/mole

Concentration of Sodium Hydroxide:                      50% (% by mass, wt/wt)

## Step 1: Calculate the volume of 100 grams of Sodium Hydroxide solution.

Formula:

Density =  weight / volume    or

Volume =  weight / density

Volume of 100 gram of Sodium hydroxide solution: 100/1.515 =  66.0066 ml

Note: 50% (w/w) Sodium Hydroxide means that 100 grams of solution contains 50 grams of Sodium Hydroxide.

The volume of 100 grams of Sodium hydroxide solution is 66.0066 ml. That means 50 grams of NaOH is present in 66.0066 ml of  Sodium hydroxide solution.

## Step 2: Calculate how many grams of NaOH is present in 1000 ml of Sodium hydroxide solution.

66.0066 ml of Sodium Hydroxide contains   = 50 grams of NaOH
1 ml of Sodium Hydroxide will contain          = 50/66.0066 grams of NaOH

1000 ml of Sodium Hydroxide will contain    = 1000 x 50/66.0066 = 757.50 grams of NaOH

1000 ml ofSodium hydroxide solution will contains 757.50 grams of NaOH.

## Step 3: Calculate number of moles of NaOH present in 757.50 grams of NaOH.

40 grams of NaOH is equal to 1 mole.
1 gram of NaOH will be equal to 1/40 moles.

757.50 grams of NaOH will be equal to = 757.50 x 1/40 = 18.9375 moles

Therefore, we can say that 1 litre ofSodium hydroxide solution contains 18.9375 moles or in other words molarity of 50% (w/w) Sodium Hydroxide is equal to 18.9375 M.