Required Practical 3

Question 1

Describe how a dilution can be calculated

Answer 1

1. Calculate dilution factor = desired concentration (C2) / stock concentration (C1)
2. Calculate volume of stock solution (V1) = dilution factor x final desired volume (V2)
3. Calculate volume of distilled water = final desired volume (V2) - volume of stock solution (V1)

Question 2

Formula for dilution

Answer 2

You can rearrange and use the formula: C1 x V1 = C2 x V2 with V2 = V1+ volume of distilled water, or:

Question 3

Worked example: Describe how you would use a 0.5 mol dm-3 solution of sucrose (stock solution) to produce 30 cm3 of a 0.15 mol dm-3 sucrose solution. (2)

Answer 3

1. Calculate dilution factor (desired concentration / stock concentration): 0.15 / 0.5 = 0.3
2. Calculate volume of stock solution (dilution factor x final volume): 0.3 x 30 cm3 = 9 cm3
3. Calculate volume of distilled water (final volume - stock solution volume): 30 cm3 - 9 cm3 = 21 cm3

Question 4

Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato)
Part 1: collecting data (steps 1-3)

Answer 4

Step 1: Create a series of dilutions using a 1 moldm-3 sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8,1.0 mol dm-3)
-Control variable: ● Volume of solution, eg. 20 cm3
Step 2: Use scalpel / cork borer to cut potato into identical cylinders
-Control variable:
● Size, shape and surface area of plant tissue
● Source of plant tissue ie variety or age
Step 3: Blot dry with a paper towel and measure /record initial mass of each piece
-Control variable:
● Blot dry to remove excess water before weighing

Question 5

Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato)
Part 1: collecting data (steps 4-5)

Answer 5

Step 4: Immerse one chip in each solution and leave for a set time (20-30 mins) in a water bath at 30oC
Control variable:
● Length of time in solution
● Temperature
● Regularly stir / shake to ensure all surfaces exposed
Step 5: Blot dry with a paper towel and measure/record final mass of each piece
-Control variable:
● Blot dry to remove excess water before weighing

Repeat (3 or more times) at each concentration

Question 6

Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato)
Part 2: processing data

Answer 6

6. Calculate % change in mass = (final-initial mass)/ initial mass
7. Plot a graph with concentration on x axis and percentage change in mass on y axis (calibration curve)
   ○ Must show positive and negative regions
8. Identify concentration where line of best fit intercepts x axis (0% change)
   ○ Water potential of sucrose solution = water potential of potato cells
9. Use a table in a textbook to find the water potential of that solution

Question 7

Explain the changes in plant tissue mass when placed in different
concentrations of solute

Answer 7

Increase in mass:
● Water moved into cells by osmosis
● As water potential of solution higher than inside cells

Decrease in mass:
● Water moved out of cells by osmosis
● As water potential of solution lower than inside cells

No change:
● No net gain/loss of water by osmosis
● As water potential of solution = water potential of cells

Question 8

Explain why the potatoes
are blotted dry before
weighing. (2)

Answer 8

● Solution on surface will add to mass (only want to measure water taken up or lost)
● Amount of solution on cube varies (so ensure same amount of solution on outside)

Question 9

Explain why % change in
mass is calculated. (2)

Answer 9

● Enables comparison / shows proportional change
● As plant tissue samples had different initial masses