Shalini and Rashid were asked to prepare 40 % solution ( mass by volume ) of potassium hydroxide ( KOH ) . Shalini prepared 40 % solution by dissolving 40 gms of KOH in 140 ml of water and Rashid prepared 40 % solution by dissolving 40 gms of KOH in little water and made the volume to 100ml . Which one of them has made the 40 % solution ? *
Answers
PREPARATION OF 40% KOH
PREPARATION OF 40% KOH
40% KOH contains 40%w/w potassium hydroxide and for 40g of the compound is containd in every 100ml of its solution.
potassium hydroxide are made as solid deliquescence pellets.
Deliquescence substances are chemical compound that tend to absorb so much water from the atmosphere that they dissolve in it to form an aqueous solution of the compound.
40%w/w KOH contains 7.14M KOH
10M KOH= 10N KOH
From
Molar Mass of each element K=39, O=16, H=1
Molar Mass of NaOH= 39+16+1
=56g
Number of Mole= Mass
Molar mass
From
40% KOH
The mass of KOH in 100ml of it solution is 40g
Thus
Number of Mole of KOH = Mass
Molar mass
Number of Mole of NaOH = 40
56
= 0.714mole
In 1000ml or 1liter of the KOH
Molar Concentration = Mass X 1000
Molar mass Volume
or
Molar Concentration = Number of mole X 1000
Volume
Molar Concentration = 40 X 1000
56 100
Molar Concentration = 7.14M
Or
Since the number of mole of 40% KOH in 100ml of the solution is 1mole
Therefore
Molar Concentration = 0.714 X 1000
100
Molar Concentration = 7.14M
Preparation Procedure
1. Make available an accurate analytical weighing balance, clean dry 500ml beaker, distilled/deionized water, stirrer, 1000ml Volumetric flask and Spatula
2. Place the beaker on the balance and tare/zero the reading
3. With the aid of the spatula weigh 400g of the KOH pellet into the clean beaker
4. Add about 400ml of distilled/deionized water and stir to dissolve the pellets in a cold water bath (Note: the reaction is exothermic because heat will be given up thereby causing increase in the temperature of the beaker, ensure you stir gently in the cold water bath)
5. When all the pellets have dissolved, transfer the solution into the 1000ml volumetric flask. Allow to cool
6. Add distilled/deionized water to the mark, shake and allow to cool
7. On cooling decrease in volume of the solution will be noticed, add distilled/deionized water again to the mark and shake.
Note:
1. KOH has high affinity for water thus this is why there is decrease in the solution after making up to the mark
2. Due to the high affinity for moisture in the atmosphere the weighing should be done in vacuum or less aerated place at a fast speed.
3. Deionized water is best for the preparation of this solution because of it lack of ions, which when present can interfere with the KOH in the solution.