Investigating water potential

Question 1

Serial dilutions

Answer 1

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Question 2

What is the first step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2

Answer 2

Fill a test tube with 10 cm’ of the 2M sucrose solution

Question 3

What is a serial dilution?

Answer 3

A serial dilution is any dilution in which the concentration decreases by the same factor in each successive step

Question 4

How do you find the dilution factor?

Answer 4

The dilution factor can be calculated using the initial volume of stock solution (Vi) divided by the final solution volume (Vt).
DF = Vi/Vf

Question 5

What is the second step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2

Answer 5

Fill 5 test tubes with 5 cm’ of distilled water.

Question 6

What is the third step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2

Answer 6

Using a pipette transfer 5 cm³ of of the sucrose solution from the test tube to the first tube of 5 cm³ of water

Question 7

What is the fouth step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2

Answer 7

• Mix the solution thoroughly. You now have 10 cm³ of sucrose solution that is half as concentrated as the solution in the stock solution (1M)
• Dilution factor is 5ml/10ml = 1/2 or 1:2 dilution

Question 8

What is the fifth step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2

Answer 8

Repeat the process by transferring 5 cm³ from the 1M solution and adding it to the next tube containing distilled water to make a 10 cm³ solution which is now half as concentrated again (0.5M)

Question 9

What is the sixth step of making five serial dilutions of a sucrose solution starting with an initial stock solution of 2M with a dilution factor of 2

Answer 9

Repeat to create 0.25M and 0.125M solutions

Question 10

What are the units for making solutions?

Answer 10

Lor dm³ = units of volume (1 dm⁻³ = 1 Litre)
ml or cm³ = units of volume (1 cm⁻³) = 1ml)
M= the unit for concentration (A 0.1M solution = 0.1 mol dm⁻³)
0.0974 M means 0.0974 mol dm⁻³ or 0.0974 mmol cm⁻³
mol dm⁻³ = unit of concentration units of moles per cubic decimetre (or
per L)
mol cm⁻³ = unit of concentration units of moles per cubic centimetre (or per ml) - To convert mol dm⁻³ 0.0974 mol dm⁻³ = 97.4 mol cm⁻³

Question 11

How would you make 15cm³ of 0.4 mol dm⁻³ sucrose solution using a stock of 1M

Answer 11

First you find the dilution factor
DF = 1M/0.4M = 2.5
So we need to make a solution which is 2.5 times weaker than the stock solution
If we had 15 cm³ of 1M to make it 2.5x weaker we would need to use 15/2.5 = 6 cm³
Then we would need to dilute it with 9 cm³ of water to keep the volume 15 cm³

Question 12

What does the method of finding a solution rely on?

Answer 12

You can apply this method to making any solution as long as you knowthe volume of the solution you want to make and the concentration of the final and original/stock solution

Question 13

Investigating water potential practical

Answer 13

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Question 14

What is the equipment list for the investigating investigating water potential practical?

Answer 14

• Potato tuber
• Cork borer
• Scalpel
• Ruler
• Distilled water
• Sucrose solution (1M)
• Boiling tubes
• Boiling tube rack
• Timer
• Digital balance
• Paper towels

Question 15

What is step 1 of the investigating water potential practical?

Answer 15

Make a series of dilutions of 1M sucrose solution. These should be at 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose. Dilute using distilled water.

Question 16

What is step 2 of the investigating water potential practical?

Answer 16

Measure 5cm³ of each dilution into separate test tubes

Question 17

What is step 3 of the investigating water potential practical?

Answer 17

• Use a cork borer to cut out six potato chips and cut down the sections into identically sized chips
  (Ensure all skin is removed as it could affect results)
• Dry each chip using a paper towel to remove excess water but do not squeeze
  (Helps remove any excess water that would affect starting mass)

Question 18

What is step 4 of the investigating water potential practical?

Answer 18

Weigh each before the start of the experiment

Question 19

What is step 5 of the investigating water potential practical?

Answer 19

Place a potato chip in each test tube (one per sucrose concentration) and leave for 20 minutes
(Must know which solution is which)

Question 20

What is step 6 of the investigating water potential practical?

Answer 20

• Remove each potato chip
• Dry gently using paper towel
  (To remove excess solution on the outside of the chip that would affect mass)
• Weigh them in turn
  (If they have gained water by osmosis, they will have increased in mass. If water has left by osmosis, they would have decreased in mass)

Question 21

What is step 7 of the investigating water potential practical?

Answer 21

• Calculate the change in mass for each sucrose solution
  (Done by subtracting the final mass from the initial mass - those that have decreased in mass will have negative results)
• Calculate the percentage change in mass for each sucrose solution
  (Means you can compare the effect on chips that don’t have the same initial mass - change in mass/starting mass = % change)

Question 22

Why is a scalpel a hazard?

Answer 22

There is a risk of there being cuts from sharp object

Question 23

What are the safety precautions for using a scalpel?

Answer 23

Cut away from fingers; use forceps to hold sample whilst cutting, keep away from the edge of the desk

Question 24

What is the hazard of broken glass?

Answer 24

There is a risk of there being cuts from sharp object

Question 25

What are the safety precautions for having broken glass?

Answer 25

Take care when handling slides and coverslips; keep glassware away from edge of desk

Question 26

How do you plot the graph for the investigating water potential practical?

Answer 26

* Plot a graph of change in mass against concentration of sucrose solution * The point at which the line of best fit crosses the x axis (zero change in mass) indicates the point at which the solution is isotonic. This is when the water potential of sucrose solution is the same as the water potential of the potato tissue, so there is no net movement of water in or out of the potato

Question 27

What is the conclusion for the investigating water potential practical?

Answer 27

* Potato chips in lower concentrations of glucose solution will increase in mass, whilst those in the higher concentrations of glucose solution will decrease in mass

Question 28

What is a calibration curve?

Answer 28

* Calibration curves are graphs used to determine an unknown concentration of a sample by comparing the unknown to a set of standard samples with known concentrations - they are also known as standard curves * A dilution series can be used to create a set of samples with known concentrations

Question 29

What can a calibration curve be used to work out?

Answer 29

* A calibration curve can be used to determine an unknown water potential in a potato sample * Water potential is the tendency of water to diffuse from one area to another * Water molecules move from areas of high water potential to areas of low water potential by osmosis * The water potential is determined by the concentration of solutes * The movement of water in and out of cells is related to the relative concentration of solutes either side of the cell membrane

Question 30

If you add a 1cm³ to 9ml of water what dilution factor do you have?

Answer 30

This is a 1 in 20 dilution or the solution has been diluted by a factor of 10, 10x more dilute than the original