Unit 2- Amount Of Substance

Question 1

How to use Avogadro constant

Answer 1

• equivalent to 1 mole of a substance (6.022×10^23)

- No. Particles = No.moles × Avogadro constant

Question 2

Calculating moles

Answer 2

• Mass in g ÷ Mr
• molar ratio is equivalent to the number of molecules in a balanced equation
• Concentration (mol/dm^-3) × volume (dm^3)

Question 3

Ideal gas equation

Answer 3

• PV=nRT
• P= pressure (Pa)
• V=Volume (m^3)
• n= No. Moles
• R=Gas constant (8.3JK^-1mol^-1)
• T= Temperature (K)

Question 4

How to write ionic equations

Answer 4

• Write balanced equation
• split dissolved ionic species into their ions
• Remove ions appearing on both sides of the equation

Question 5

How to calculate and use empirical formulae

Answer 5

• calculate Mr of all atoms in a molecule and find the ratio by dividing all of them by the smallest Mr
• Write the ratio as a formula
• use this to find the real formula by dividing the molecules Ar by the empirical Ar and multiply all the values of the ratio by that constant

Question 6

Making a standard solution for titration

Answer 6

• Dissolve a known amount of solid in a known amount of water to create a known concentration
• calculate mass of solid necessary by calculating moles necessary and then the mass you need to weigh out
• After weighing, tip the solid into a beaker, then re-weigh the weighing bottle to see if there is any residue to subtract from the mass added
• add distilled water to the beaker to dissolve the solid, and pour that solution into another beaker of the required volume
• rinse the original beaker with distilled water to remove residue and add that to the second beaker too
• top up the second beaker with distilled water to the exact volume required (make sure botton of meniscus is exactly on the line)
• mix well and calculate the exact concentration

Question 7

Titrations

Answer 7

• fill a burette with a standard solution you know the concentration of
• fill a beaker below it with a fixed volume of a substance you don’t know the concentration of, and a suitable indicator
• take a rough initial reading to get an idea of the end point
• take at least three serious readings of the exact amount needed to change the indicator to find an average titre
• the average titre gives you the volume of the standard solution used in a balanced equation, allowing you to calculate the unknown concentration from the moles of the standard solution

Question 8

Chemical yield

Answer 8

• theoretical yield is the moles of product calculated to be made, using a balanced equation and molar ratio
• percentage yield is the actual yield over the theoretical yield
• this shows how much product is lost through incomplete reactions or transfer between containers

Question 9

Atom economy

Answer 9

• How much of the product is what you want over the total mass of all products
• (molecular mass of desired product/sum of molecular masses of all reactants) *100
• economically advantageous to have high atom economy as less money is wasted on reactants, less waste products are made that need disposing
• advantageous for society as drugs are cheaper when produced with high efficiency
• high atom economy is better for the environment as less harmful waste, more sustainable processes