>>3.1 - Amount of Substance✔

Question 1

When doing a titration, what must go in the burette and what must go in the conical flask?

Answer 1

• Solution of known concentration in burette.
• Solution of un-known concentration in conical flask.

Question 2

What affect will adding water to a solution have?

Answer 2

• Changes its volume and concentration.
• However the number of moles of the original chemical remains the same.

Question 3

What is the definition of concentration?

Answer 3

• The amount of solute dissolved in 1dm³ of solution.
• moldm⁻³
• Described as the molarity of a solution.

Question 4

Errors

What affect does leaving the filter funnel in have?

Answer 4

• Leftover acid in funnel may drip into burette.
• Lowers titre volume.

Question 5

Errors

What affect does having an air bubble under the tap have?

Answer 5

• Incorrect volume measured - air bubble will fill during titration.
• Increases titre volume.

Question 6

Errors

What affect does not using a conical flask have?

Answer 6

• Acid may spill out when swirling.

Question 7

Errors

What affect does not swirling have?

Answer 7

• The acid isn’t evenly distributed.
• Increases titre volume.

Question 8

Errors

What affect does not washing the sides of the conical flask before end point have?

Answer 8

• Acid could be on the sides and not in the main solution, washing the sides with distilled water doesn’t affect moles of either reactant.
• Increases titre volume.

Question 9

What is the equation for uncertainty in a titration?

Answer 9

• (error value/quantity measured)x100
• If measuring twice, double the uncertainty.
  e.g: 25cm³ measuring cylinder with +-0.5
  (0.5/25)x100=2%

Question 10

What three factors add up to give the total error in a titre?

Answer 10

• Reading burette at the start.
• Reading burette at the end.
• Judging the end point to within one drop.

Question 11

Give one solution to improving % error of a titration:

Answer 11

• Use a more dilute solution in the burette to increase volume added so % is smaller when calculating the uncertainty.

Question 12

What must chemical reactions be before being used in calculations and why?

Answer 12

• Must be balanced.
• This is because the reacting ratios must be correct.

Question 13

What is the equation for % yield?

Answer 13

• (actual mass/theoretical mass)x100

Question 14

What is the equation for atom economy?

Answer 14

• (Mr of desired product/Mr of reactants)x100
• Must include the ratio in the Mr.

Question 15

In industry why is it desirable to have a high atom economy in a chemical process?

Answer 15

• There is little or no waste product therefore it means the process is more economically viable for industry scale manufacture.
• Having a high atom economy is also beneficial for the environment as it uses less natural resources and therefore often uses less energy.

Question 16

What is atom economy?

Answer 16

• A measure of the proportion of reactant atoms that become part of the desired product in the balanced chemical equation.

Question 17

What is 1dm³ in cm³?

Answer 17

• 1000cm³

Question 18

What is 1m³ in dm³?

Answer 18

• 1000dm³

Question 19

How do you convert from moldm⁻³ to gdm⁻³?

Answer 19

• Multiply moldm⁻³ by the Mr.

Question 20

What is the definition of Ar?

Answer 20

• Relative atomic mass is defined as the average mass of an atom of an element relative to 1/12th the Ar of an atom of carbon-12.

Question 21

What is the definition of Mr?

Answer 21

• Relative molecular mass is defined as the average mass of a molecule of a compound relative to 1/12th the Ar of an atom of carbon-12.

Question 22

What term is used for the mass of ionic compounds?

Answer 22

• Relative formula mass.

Question 23

What is the equation for diluting a solution?

Answer 23

• C₁xV₁=C₂xV₂

Question 24

What is avogadro’s constant?

Answer 24

• 6.022x10²³ particles in 1 mole.
• Allows the number of particles present in a sample of a substance with known mass to be found.

Question 25

What is the number of elementary entities equal to?

Answer 25

• Number of moles x Avogadro’s constant

Question 26

What are the three equations for moles?

Answer 26

• Mass/Mr
• Concentration x Volume
• Volume/24dm³ pV=nRT

Question 27

What is the definition of a mole?

Answer 27

• The number of atoms in 12g of carbon-12.

Question 28

What can the mole be applied to?

Answer 28

• Electrons, atoms, molecules, ions, formulas and equations.

Question 29

What is the ideal gas equation?

Answer 29

• pV=nRT
• p=pressure (Pa)
• V=volume (m³)
• n=moles (mol)
• R=gas constant (8.314JK⁻¹mol⁻¹)
• T=temperature (K)

Question 30

What is the normal atmosphere pressure roughly equal to?

Answer 30

• 101kPa (1atms.)

Question 31

How do you convert from °C to kelvin?

Answer 31

• Add 273.
• To convert back to °C minus 273.

Question 32

What is the empirical formula?

Answer 32

• Simplest whole number ratio of atoms of each element in a compound.

Question 33

What is the molecular formula?

Answer 33

• The true ratio of atoms of each element in a compound.

Question 34

Empirical Formula/Molecular Formula:

Example: 2.84g of phosphorus oxide contains 1.24g of phosphorus, what is the empirical formula of this compound:

Answer 34

• P₂O₅
• You work out the moles of each element then you divide the moles of each element by the smallest moles present to get a ratio.
• In this case the ratio isn’t a whole number so we have to multiply both by two to get the empirical formula.
• P
  Mass: 1.24g
  Mr: 31
  Moles: 0.04
  Ratio: 1
• O
  Mass: 1.60g
  Mr: 16
  Moles: 0.1
  Ratio: 2.5

Question 35

What are hydrated compounds?

Answer 35

• These are compounds that have water bonded to them, this affects the Mr of the compound.

e.g: CuSO₄.5H₂O (5 waters bonded to the salt.)

Question 36

How can you find the mass of the solid in a hydrated compound?

Answer 36

• The water can be removed by heating until no mass change is observed, and then we can measure the mass of water that is lost and the mass of the solid remaining.

Question 37

deal Gas Equation:

Example: 30cm³ of methane reacts with 10cm³ of oxygen forming CO₂ and water. What will the total volume of the reaction be when the reaction is complete?

(assume 298K, 100kPa throughout)

Answer 37

⓵ CH₄ + 2O₂ → CO₂ + 2H₂O (we don’t know if the moles are balanced)

⓶ Find the moles of each reactant using the equation:

CH₄

100000x(30x10⁻⁶)/8.314x298=1.21x10⁻³

O₂

100000x(10x10⁻⁶)/8.314x298=4.03x10⁻⁴

↓

Limiting Factor as from the equation we know that 4x10⁻⁴ moles of O₂ can react with only 2x10⁻⁴ moles of methane, therefore we have excess methane (1.21x10⁻³) so oxygen will run out first.

⓷ Refer to table

⓸ Total up all the final moles and put into the equation:

V=(1.6x10⁻³)(8.314)(298)/100000= 3.96x10⁻⁵m

Question 38

What is the equation for Avogadro’s constant, L?

Answer 38

• Mr/Mass