Moles II - 5.1

Question 1

How to get from cm cubed –> dm cubed?

Answer 1

divide by 1000

Question 2

How to get from dm cubed –> cm cubed?

Answer 2

x 1000

Question 3

How to calculate the moles of a gas?

Answer 3

moles = volume/24

Question 4

What is the unit for the concentration of a solution?

Answer 4

mol/dm3

Question 5

What is the formula for calculating the concentration of a solution?

Answer 5

concentration = moles of solute / volume of solvent

Question 6

What is the moles equation when concentration is involved/

Answer 6

moles = concentration x volume

Question 7

What is titration?

Answer 7

Titration is a practical method used to measure the volume of acid needed to neutralise a particular volume of alkali, or vice-versa.

Question 8

What are the two special pieces of equipment used in titrations?

Answer 8

Volumetric pipette - measures out 25 cm3 very accurately

Burette - measures any volume to the nearest 0.05 cm3

Question 9

How do you set up a titration?

Answer 9

1. Use a volumetric pipette to place 25 cm3 of the unknown alkali into a conical flask.
2. Add a few drops of either methyl orange or phenolphthalein indicator to the flask. Universal indicator doesn’t work because it changes colour too gradually.
3. Fill a burette with known acid and clamp it above the conical flask.

Question 10

How do you use a titration?

Answer 10

4. Measure the initial volume of known acid on the burette, to the nearest 0.05 cm3
5. Use the burette to add acid into the conical flask drop-by-drop and swirling the flask.
6. Close the burette tap as soon as the colour in the conical flask changes.
   The start colour will be the alkaline colour, the end colour will be the acidic colour (because the last drop of acid always has a tiny amount more than is
   needed to neutralise).
7. Measure the final volume of known acid on the burette, to the nearest 0.05 cm3
8. Calculate titre (final volume − initial volume).
9. Repeat to get concordant (within 0.2 cm3) titres

Question 11

What does the term concordant mean?

Answer 11

within 0.2

Question 12

Easiest way to calculate the results:

Answer 12

1. Grid out the numbers in the question (conc and vol)
2. Use your grid to help you do a normal reacting moles calculation:
3. Calculate the moles of H2SO4.
4. Use the mole ratio from the equation to get the moles of NaOH.
5. Calculate the concentration of the NaOH