Amount of Substance 2

Question 1

how do you calculate concentration and what are the units

Answer 1

concentration = moles / volume

conc = mol dm-3
volume is in dm3/cm3

Question 2

how do you convert from dm3 to cm3

Answer 2

1dm3 = 1000cm3

cm3 / 1000 = dm3

Question 3

how do you convert from mol dm-3 to g dm-3

Answer 3

multiply by RFM

e.g 1 mol dm-3 of NaOH
1 x 40 = 40 g dm-3

Question 4

what is the procedure for a titration

Answer 4

• rinse burette with acid to prevent any dilution
• using a funnel, add acid to the burette under the zero line
• take a reading from the bottom of the meniscus
• using a pipette filler transfer 25cm3 of alkali into a conical flask
• add 2 to 3 drops of indicator to the flask
• place the flask on a white tile
• constantly swirl the flask whilst adding acid drop by drop until end point is reached
• perform a rough titration first, then an accurate one

Question 5

why is it important to fill the space below the tap in the burette with solution before beginning a titration

Answer 5

• space will fill during titration
• space in the tap counts to the final volume and so volume measured would be too high

Question 6

why is a conical flask used instead of a beaker

Answer 6

mixing and swirling solution in a conical flask is easier than in a beaker as there is less risk of contents spilling due to the neck of the flask

Question 7

how does rinsing improve the accuracy of the end point

Answer 7

makes sure all of the acid and alkali can react properly in the flask, as the solution tends to travel up the sides of the flask

Question 8

explain why the water used for rinsing has no effect on the accuracy of the titre

Answer 8

water is not a reagent and so wont affect the moles of each solution

Question 9

how does repeating a titration make the vaue of the average titre more reliable

Answer 9

a single titration may be anomolous and repeating a titre should bring you closer to the end point

Question 10

how do you calculate a percentage uncertainty

Answer 10

uncertainty x 2 / average titre x 100

Question 11

how is room temperature pressure related to moles

Answer 11

1 mole of any gas at room temperature and pressure occupies a volume of 24dm3

Question 12

what is the ideal gas equation

Answer 12

PV = nRT

pressure x volume = number of moles x gas constant x temperature

Question 13

what are the conversions needed for the gas equation

Answer 13

temp = °C +273 = K
pressure = KPa x 1000 = Pa
volume = cm3 / 1 000 000 = m3