2 Amount of substance

Question 1

What is the equation that links moles, mass and Mr

Answer 1

Moles=mass/mr

Question 2

What is the equation that links moles volume and concentration

Answer 2

Concentration=moles/volume

Question 3

Describe what is meant by Avogadro’s constant

Answer 3

• The number of particles / atoms / ions in 1 mole of a substance
• 6.022x10^23
• number of particles= moles X 6.022x10^23

Question 4

What is the ideal gas equation

Answer 4

PV=nRT

Question 5

How do you convert °C into K

Answer 5

+273

Question 6

Describe what is meant by the molecular formula of a compound

Answer 6

The actual number of each atom of an element in a compound

Question 7

Describe what is meant by the empirical formula

Answer 7

• The simplest whole number ratio of each atom of an element, in a compound

Question 8

Describe what is meant by atom economy and write the formula

Answer 8

• The percentage of reactants that become useful products
• Atom economy(%) = mass of desired product/total mass of product X100

Question 9

What is the atom economy when there is only one product in the equation

Answer 9

100%

Question 10

Describe how to calculate percentage yield

Answer 10

mass of product actually made/maximum theoretical mass X 100

Question 11

Why is the percentage yield never 100%

Answer 11

• Not all the reactants are being used up
• Products escaping from the reaction vessel
• Physical loss of products during transfer

Question 12

Describe how to prepare a standard solution
(RP1) A

Answer 12

• Transfer an accurate mass of solid into a weighing boat and measure the mass using a top pan balance
• Transfer the solid into a clean, dry beaker and weigh the empty weighing boat
• Calculate the difference in mass
• Add 100cm3 of distilled water to the same beaker, to dissolve the solid, stirring with a glass rod
• Transfer the contents in the beaker to a 250 cm3 volumetric flask using a funnel
• Rinse the beaker, funnel and glass rod with distilled water, using a wash bottle
• Transfer the washing to the same 250cm3 volumetric flask
• Fill the volumetric flask with distilled water up to the 250cm3 meniscus, at eye level.
• Place a stopper on the flask and invert several times to mix thoroughly, so the concentration is the same throughout the flask,

Question 13

Describe how to carry out titration
(RP1) B

Answer 13

• Rinse the burette with the standard solution
• Discard the rinsings and fill the burette with the standard solution to the 0cm3 meniscus
• Rinse a pipette with the sodium hydroxide solution
• Using a pipette and pipette filler, transfer 25cm3 of this solution into a conical flask
  Add 2-3 drops of phenolphthalein indicator to the conical flask
• Add the standard solution from the burette, with constant swirling, until the indicator just changes colour. This is a rough titration.
• Repeat the steps above several times to achieve 2-3 concordant results, adding the solution dropwise near the endpoint
• Calculate the mean titre from concordant results

Question 14

Describe how to calculate uncertainty

Answer 14

(Uncertainty / quantity measured ) x 100

Question 15

Describe how to calculate uncertainty if 2 measurements are taken

Answer 15

(Uncertainty x 2 / quantity measured ) x 100

Question 16

Explain what the effect of the titre would be if the pipette used to transfer the acid solution was filled to slightly above the mark.

Answer 16

Increased titre because more moles of acid are transferred to the conical flask

Question 17

Explain what the effect of the titre would be if the pipette used to transfer the acid solution was filled to slightly below the mark.

Answer 17

Decreased titre because less moles of acid are transferred to the conical flask

Question 18

Explain what the effect of the titre would be if the volumetric flask was filled to slightly above the mark

Answer 18

Decreased titre because the concentration of the acid is lower

Question 19

Explain what the effect of the titre would be if the volumetric flask was filled to slightly below the mark

Answer 19

Increased titre because the concentration of the acid is higher

Question 20

A student suggests washing out the pipette with water before filling it with acid solution.
Explain why this is not a good idea.

Answer 20

Water left in the pipette would dilute the acid, therefore, the titre will decrease