CHAPTER 7: QUANTIFYING ATOMS

Question 1

what standard are all masses compared to

Answer 1

all masses are compared to an atom of the common isotope of carbon → carbon 12 or 12C

Question 2

relative isotopic mass

Answer 2

is the mass of an atom of a particular isotope relative to the mass of an atom of carbon-12

Question 3

relative isotopic abundance

Answer 3

is the percentage abundance of an isotope in the natural environment

Question 4

relative atomic masses

Answer 4

• the relative atomic mass (Ar) of an element is the WEIGHTED AVERAGE of the relative masses of the isotopes of the element on the 12C scale
• formula:

Ar= relative isotopic mass 1 x % abundance + relative isotopic mass 2 x % abundance ….

• can continue on depending on how many isotopes there are

Question 5

calculating percentage abundance

Answer 5

• say the percentage abundance of a particular type of isotope is x
• then the other isotope would be 100 -x
• and sub those into the relative atomic mass formula to find x

Question 6

relative molecular mass

Answer 6

• relative molecular mass Mr, is equal to the sum of the relative masses of the atoms in the molecule
• eg CO2
  
  • Ar(C)= 12
  • Ar(O)= 16
  • Mr= 1x12 + 2x16 = 44

Question 7

relative formula mass

Answer 7

for compounds that exist as lattices
eg. NaCl

Question 8

what is the mol

Answer 8

counting unit of chemistry given the symbol n and the unit mol

Question 9

avogadro’s number

Answer 9

1 mole of any substance is defined as the same number of particles as there are atoms in exactly 12 grams of carbon 12

Question 10

molar mass

Answer 10

is the relative atomic mass or relative molecular mass or relative formula mass express as g/mol

Question 11

percentage composition

Answer 11

• the proportion of each element expressed as a percentage of the total mass of the compound
• % by mass of an element = mass of the element in 1 mol of compound/ molar mass of the compound x 100

Question 12

empirical formula

Answer 12

the empirical formula of a compound gives the simplest whole number ratio of elements in that compound

Question 13

molecular formula

Answer 13

• the molecular formula gives the actual number of atoms of each element present in a molecule, rather than the simplest whole number ratio
  
  • can be the same or different to empirical formula