Practical chemistry CBT

Chemistry Lecture Quiz

Theory Questions (15)

1. Define concentration and explain its units.

Concentration measures the amount of solute per unit volume of solution, commonly expressed in mol/L (Molarity) or g/L.

2. What is the difference between strong and weak acids?

Strong acids ionize completely in solution; weak acids ionize partially.

3. Explain the role of indicators in titrations.

Indicators show the end point of a titration by changing color when the reaction is complete.

4. When would you use methyl orange instead of phenolphthalein?

Use methyl orange for strong acid-strong base titrations or strong acid-weak base, because it changes color in acidic pH.

5. What are the color changes of methyl orange in acid and base?

Acid: Red, Base: Yellow.

6. What are the color changes of phenolphthalein in acid and base?

Acid: Colorless, Base: Pink.

7. What is the purpose of a rough titration?

A rough titration determines approximately the volume of titrant needed before performing accurate titrations.

8. Explain how to calculate the average titre.

Average titre = sum of accepted titres ÷ number of accepted titres, only using titres within ±0.2 cm³ difference.

9. Why should the difference between two accepted titres be within ±0.2?

To ensure accuracy and precision; larger differences indicate experimental error.

10. Explain the difference between primary and secondary standard solutions.

Primary standards are pure and stable; secondary standards are standardized against primary standards.

11. How does temperature affect titration reactions?

Temperature can change reaction rate and solubility, affecting the endpoint accuracy.

12. Define molarity and normality.

Molarity (M) = moles of solute per liter; Normality (N) = equivalents of solute per liter.

13. What is the significance of the end point in titration?

The end point indicates the reaction is complete and allows calculation of concentrations.

14. Describe the procedure for preparing a standard solution.

Weigh pure solute accurately, dissolve in solvent, and dilute to a known volume in a volumetric flask.

15. Why is it important to rinse the burette before titration?

To remove impurities and ensure the titrant does not get diluted or contaminated.

Calculation Questions (10)

1. Calculate the molarity of a solution when 25cm³ of 0.1M NaOH neutralizes 50cm³ of HCl.

Molarity of HCl = (0.1 × 25)/50 = 0.05 M

2. Determine the concentration of H2SO4 if 30cm³ reacts with 20cm³ of 0.2M NaOH.

H2SO4 + 2NaOH → Na2SO4 + 2H2O. M1V1 = 2M2V2 → M1 = (0.2 × 20 × 2)/30 = 0.267 M

3. Calculate the number of moles in 0.5L of 2M HCl.

Moles = M × V = 2 × 0.5 = 1 mole

4. Find the mass of NaOH required to prepare 250cm³ of 0.1M solution.

Moles = 0.1 × 0.25 = 0.025 moles. Mass = 0.025 × 40 = 1 g

5. Determine the volume of 0.2M HCl needed to react with 0.1g of Na2CO3.

Moles Na2CO3 = 0.1/106 ≈ 0.000943. HCl + Na2CO3 → 2HCl per 1 Na2CO3 → V = (0.000943×2)/0.2 ≈ 0.00943 L = 9.43 mL

6. Calculate molarity from given titration data: 25cm³ acid, 0.1M base, titre = 23.5cm³.

M1V1 = M2V2 → M1 = (0.1×23.5)/25 ≈ 0.094 M

7. Find the concentration of acetic acid if 50cm³ reacts with 25cm³ of 0.1M NaOH.

M1V1 = M2V2 → M1 = (0.1×25)/50 = 0.05 M

8. Determine moles of HCl in 100cm³ of 0.5M solution.

Moles = 0.5 × 0.1 = 0.05 moles

9. Calculate the titre average if three accepted values are 24.1, 24.3, and 24.2cm³.

Average = (24.1 + 24.3 + 24.2)/3 = 24.2 cm³

10. Find the normality of H2SO4 if 25cm³ reacts with 20cm³ of 0.1N NaOH.

H2SO4 + 2NaOH → N1V1 = 2N2V2 → N1 = (2×0.1×20)/25 = 0.16 N

Titration Tables (5)