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A chemist determined the concentration of a bleach solution containing NaClO by volumetric analysis - Leaving Cert Chemistry - Question 1 - 2011
Question 1
A chemist determined the concentration of a bleach solution containing NaClO by volumetric analysis. A 25.0 cm³ sample of the bleach was first diluted to exactly 500... show full transcript
Worked Solution & Example Answer:A chemist determined the concentration of a bleach solution containing NaClO by volumetric analysis - Leaving Cert Chemistry - Question 1 - 2011
Step 1
Describe how the 25.0 cm³ sample of the original bleach solution was diluted to exactly 500 cm³.
Answer
To dilute the 25.0 cm³ sample of bleach to 500 cm³, follow these steps:
- Using a pipette, transfer precisely 25.0 cm³ of the original bleach solution into a 500 cm³ volumetric flask.
- Add deionized water or distilled water to the flask. Start by adding water dropwise or using a wash bottle to ensure accuracy.
- Fill the flask with water until the meniscus reaches the 500 cm³ mark at eye level.
- Invert the flask multiple times to mix thoroughly, ensuring a homogeneous solution throughout with consistent concentration.
Step 2
Step 3
Give two reasons why this indicator was used.
Answer
- The starch indicator was used because it reacts with iodine to give a distinct blue-black colour, making it easier to see the endpoint of the titration.
- This indicator helps to identify when all the iodine has reacted, allowing for accurate titration measurement.
Step 4
What was the purpose of standing the conical flask on a white tile during the titrations?
Answer
The purpose of placing the conical flask on a white tile during titration is to enhance visibility of the colour change. A white background allows for clearer observation of any colour developments or endpoint distinctions.
Step 5
Step 6
Calculate the concentration of NaClO in moles per liter (M) in the original bleach.
Answer
To calculate the concentration of NaClO,
- Using the average volume of Na2S2O3 necessary for the reaction (16.1 cm³ or 0.0161 L), apply the formula:
ext{M} = rac{n}{V} where n is the number of moles and V is the volume in liters. 2. The amount of NaClO reacts with Na2S2O3 in a 1:1 mole ratio, so the number of moles of NaClO is the same as that of Na2S2O3 calculated from:
ext{M} = rac{0.10 imes 0.0161}{1} = 0.00161 ext{ moles} 3. Given that the original bleach sample was diluted from 25.0 cm³ to 500 cm³, multiply the concentration by the dilution factor (20):
Step 7
What was the concentration of NaClO in the original bleach in grams per liter?
Answer
To find the concentration of NaClO in grams per liter:
- Use the molar mass of NaClO, which is approximately 74.44 g/mol.
- Multiply the concentration in moles per liter (0.0322 M) by the molar mass:
3. Thus, the concentration of NaClO in grams per liter is approximately 2.40 g/L.
Step 8
What was the concentration of NaClO in the original bleach as a % (w/v)?
Answer
To calculate the concentration as a % (w/v):
- Use the formula:
ext{Percentage (w/v)} = rac{ ext{grams of solute}}{ ext{volume of solution (mL)}} imes 100 2. For NaClO:
rac{2.40 ext{ g}}{1000 ext{ mL}} imes 100 = 0.24 ext{% (w/v)} 3. The concentration of NaClO in the original bleach is therefore 0.24% (w/v).