7 — mole concept and stoichiometry

Question 1

Relative atomic mass

Answer 1

Relative atomic mass (Ar) of an elemen is the average mass of one atom of that element relative to 1/12 the mass of an atom of carbon-12

Question 2

Relative molecular mass

Answer 2

Relative molecular mass (Mr) of a molecular substance is the mass of one molecule of that substance relative to 1/12 the mass of an atom of carbon-12

Question 3

Molar mass

Answer 3

Molar mass is the mass of one mole of a substance

Mr = Ar of the element or Mr of the compound

Question 4

Empirical formula

Answer 4

The simplest ratio of elements in a compound

Molecular formula is a multiple of the empirical formula

Do the working as table below

Element:
% by mass or mass:
Ar:
No. Of moles/mol (% by mass divide by Ar):
Mole ratio (divide by smallest of mole value):

Question 5

Titration

Answer 5

Titration
1. Fill a clean 50.00cm³ burette with the titrant.
2. Add a fixed volume of analyte, typically 25.0cm³, to a clean conical flask using a pipette
3. Add a few drops of indicator to the conical flask
4. Slowly add the titrant from the burette into the conical flask. Swirl the flask constantly as the titrant is added. Stop adding titrant when contents of the flask has changed colour permanently, which is the end point of the reaction. Record the volume of titrant used.
5. Repeat steps 1 to 4 without adding the indicator.

Question 6

Analysis of molecule X shows that it contains, by mass, 52% carbon, 13% hydrogen and 35% oxygen. Which other info is needed to determine the molecular formula?

Answer 6

Relative atomic masses of carbon, hydrogen and oxygen and relative molecular mass of X.

Question 7

Explain why the molecular and empirical formulae of ethanol are identical

Answer 7

The molecular formula for ethanol cannot be reduced further into its lowest terms in order to obtain the empirical formula.

Question 8

Suggest why the percentage yield is not 100%

Answer 8

Reactants may be impure and contains impurities burnt in air and escaped as gas

OR (depending on qs):
Reactant 1 reacts with reactant 2 to form [extra compound] thus leaving lesser reactants to form [desired product]

OR
Reaction is reversible so some of the [product] decomposes back to form [reagent]

Question 9

The difference in molecular mass of Cl2 and Br2 is not enough o account for the difference in densities. Explain why. [2]

Answer 9

Though the Mr of Br2 is about double of Cl2, (quote) [1] the density increases by more than 100 times [1]. Thus molecular mass is not enough to account for the difference in densities.

Question 10

Explain why the relative atomic mass of cobalt is closer to the mass number of Co-59. [1]

Answer 10

There is a higher relative abundance of Co-59 as compared to nickel.