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The Mole Flashcards

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1
Q

What is relative isotopic mass and why is it used? what is an example using hydrogen? does it have units? What is the symbol?

A

Atoms are too small to be weighed individually
Instead the mass of one isotope is compared to that of another
Carbon- 12 is used as the standard. It is given the mass of 12
All other elements are relative of carton-12
Since the masses are relative, there are no units
H (1) is approximately 1/12 the mass of C-12
Ir

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2
Q

what is the most accurate way of evaluating the mass of an atom? how does this work?

A

The most accurate way of evaluating mass of an atom is to use the mass spectrometer
It separates individual isotopes in a sample of an element and determines the mass of each isotope, relative to C-12 and their relative abundances

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3
Q

what information does a mass spectrum provide?

A

Provides information about:
The number of isotopes in a given sample
The relative isotopic mass of each isotope
Relative abundance of isotopes

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4
Q

What is relative atomic mass? what is it used for? Why is it needed? where can this be found?

A

Most elements are made up of more than one isotope
Naturally occurring elements are a mixture of isotopes
Not just one isotope is useful in calculations
It is more useful to have an average mass which takes into account the relative atomic abundances of isotopes
This is called a weighted average
In C-12 this is assigned 12 and is termed its relative atomic mass (Ar)
Relative atomic mass can be found on your periodic table

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5
Q

what is the formula for relative atomic mass?

A

Ir (1) x abundance (1) + Ir (2) x abundance (2) + so on
Ar = ——————————————————————
100

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6
Q

how do you calculate the percentage abundance from a mass spectrum?

A

Measure the peaks with a ruler, not from the x axis but the bottom of the peaks
Add these scores up
Divide the total by each individual score
Multiply these scores by 100
These are your percentages of abundance and should add up to 100

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7
Q

How do you calculate the percentage abundance when you know the Ar and the Ir but not the abundance?

A

Let Ar equal the number given
Let the relative atomic masses equal the numbers given
Let one % abundance be x and the other 100-x

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8
Q

What is relative formula mass?

A

The relative formula mass, Mr, is defined as the sum of the relative atomic masses of the atom as given by the chemical formula of a species
Relative formula mass applies to molecular and non-molecular substances such as ionic compounds

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9
Q

what is the relative molecular mass?

A

The relative molecular mass, Mr, is defined as the mass of a molecule relative to C-12, which is assigned as mass of 12
Relative molecular mass only applies to molecular substances

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10
Q

how are both relative formula and molecular masses found and what is the symbol for each? Which do you use?

A

Both are given the symbol Mr and the relative formula mass is a more general term that can be used in place of relative molecular mass
Both are found by adding up the atomic masses of atoms as given by the formula of the substance

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11
Q

What is the mole? Why is it used? what is the actual number of the mole?

A

Chemical reactions always involve large numbers of atoms, ions or molecules
To avoid having to compare large numbers the mass of a number of particles has been determined and measurements of masses are scaled accordingly
Chemists use the mole to compare the amounts of reactants and products
The mole is a counting unit like a dozen
The number of objects in a mole is 6.02x10^23

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12
Q

where did the number, mole, come from? What symbol is it given?

A

A mole of a substance is the amount of the substance that contains as many specified particles as there are atoms in exactly 12g of C-12
This number is called Avogadro’s number or constant and has the symbol NA

1 mole of particles = 6.02x10^23 = the number of atoms in 12g of C-12

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13
Q

what are the units and symbols for mole?

A

Amount of substance (mole)
Symbol- n
Unit- mol

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14
Q

should Avogadro’s number determine the number of sig figs?

A

no

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15
Q

what is the formula for converting the number of particles into number of moles?

A

Converting number of particles to number of moles

                             Number of particles  Number of moles = ———————— 
                             Avogadro’s number

           N
   n = ——
           NA
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16
Q

what is the formula for converting number of moles into number of particles?

A

Number of particles = Number of moles x Avogadro’s number

N = n x NA

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17
Q

how do you work out how many atoms there are in an amount of mol?

A

Use N = n x NA
Number of mol x Avogadro’s number
Simplify and round your answer

18
Q

how do you work out how many molecules there are in an amount of mol?

A

Use N = n x NA
Number of mol x Avogadro’s number
Simplify and round your answer

19
Q

How do you work out how many moles are in a number of atoms?

A
  1. Use
            N
n = ——
        NA
  2. Divide the number of atoms by Avogadro’s number
  3. Simplify and round your answer
20
Q

what is the rule when converting between molecules and atoms? What are some examples?

A

Ex. 1 mole of oxygen atoms = 6.02 x 10^23 oxygen atoms

Ex. 1 mole of hydrogen molecules (H2) = 2 moles of hydrogen atoms (H)

Ex. 10 moles of C6H12O6 = 60 moles of C atoms, 120 moles of H atoms, 60 moles of O atoms

21
Q

How do you work out the number of atoms in a substance in a compound sample? ex CH4 contains 3.18 x 10^22 molecules

A
  1. Firstly calculate the number of moles of the compound in the sample. Use n = N/NA which would be 3.18 x 10^22 divided by 6.02 x 10^23
  2. Your answer in 5.28 x 10^-2
  3. Then calculate the number of say H (in moles) in the sample. To do that multiply the previous answer (5.28 x 10^-2) by 4 because there are 4 parts H in the formula
  4. Your answer is 2.11 x 10^-1
  5. Then calculate the number of H atoms in the sample by using N = n x NA
  6. n is the number of moles calculated before
  7. The answer is 1.27 x 10^23
22
Q

what is molar mass? what is the symbol? how can it be calculated? what is the unit?

A

The mass of one mole of substance is called molar mass.
The symbol is M.
It can be found by calculating the relative formula mass (Mr).
The molar mass is equivalent to taking the relative formula mass measured in gmol^-1.
Unit is gmol^-1

23
Q

what is the formula for converting mass to number of moles?

A

Mass
Number of moles = —————
Molar Mass

              m(g) n(mol) = ————
           M(g/mol)
24
Q

what is the formula for converting number of moles to mass?

A

Mass = number of moles x molar mass

m(g) = n(mol) x M(gmol^-1)

25
Q

how do you find the mass of a molecule when you know the number of mol?

A

To find M times the relative formula mass of the atom by 2

Then use m(g) = n(mol) x M(gmol^-1)

26
Q

how to find the mass of a compound?

A

To find M, add up all of the elements like you would with relative formula masses

27
Q

what are the two ways of finding number of moles?

A

m(g)
n(mol) = ————
M(g/mol)

or

      N   n = ——
      NA
28
Q

how do you find the mass if you are given N (number of particles) and M (molar mass)

A

First find n using the n = N/NA formula

Then with the number of moles you can use m = n x M

29
Q

How do you find how many atoms there are from a certain amount (in grams) of an element?

A

You need to find N
You have m so write it out
Calculate M using relative formula mass and the periodic table
Calculate n = m/M to find the number of moles
With the number of moles you can find N using the N = n x NA formula

30
Q

what is percentage composition? What are the two ways it can be determined?

A

The percentage composition, by mass, of a compound indicates what proportion fo the total mass of the compound is accounted for by the mass of each element
It can be determined using experimental results involving masses or from the chemical formula

31
Q

how can you find percentage composition using experimental results involving masses?

A

m(element)
% Element = —————— x 100%
m(compound)

This takes less time if you just work out the last element by adding up the others and subtracting them from 100

32
Q

how can you work out percentage composition using chemical formula?

A

Mass of element in 1 mole of the compound
% Element = —————————————————— x 100%
M(compound)

Mass of element in 1 mole of the compound basically means M (element)

33
Q

what is a compound? what is the empirical formula? what is the molecular formula? can these be the same?

A

A compound is a pure substance that consists of two or more elements chemically combined
Empirical formula is the formula that gives the simplest whole number ratio of atoms (or ions for ionic species) in a pure substance
The molecular formula gives the actual number of atoms present in a molecule. This is used exclusively for covalent molecular species
Sometimes the empirical and molecular formula are the same

34
Q

how can you determine the empirical formula for metallic elements, ionic substances and covalent substances?

A

For metallic elements, the empirical formula is the symbol of the element
For ionic substances it is the same as the ionic formula eg. BaCO3
For covalent substances the molecular formula may be the same as the empirical formula or a simple whole number ratio of the empirical formula
When determining the empirical formula of a substance you always need to find the simplest ratio of number of moles in the substance
When the mass of each element is known, then the number of moles of each element can be calculated and hence the ratio of the number of moles and the empirical formula

35
Q

how do you find the empirical formula?

A
  1. Write out the elements as a ratio eg. C : O
  2. Write out the mass (g) under each as a ratio. If it is a percentage treat it as a mass in 100g
  3. Write out (again as a ratio underneath) m/M
  4. This finds the number of moles and write this underneath as a ratio
  5. Then divide each by the smallest, one will aways be one
  6. This gives the simplest mole ratio (still written as a ratio underneath)
  7. And then put that together as the empirical formula
  8. If it isn’t a whole number, try multiplying by 2
36
Q

how can you find the molecular formula with the empirical formula and Mr?

A

The molecular formula can be found from the empirical formula if the relative molecular formula or molar mass is known
These questions ask you to find the relative empirical formula mass or EFr
This is found by adding together the relative atomic masses of the atoms in the empirical formula

37
Q

an example of finding the molecular formula. Determine the molecular formula of benzene which has the empirical formula CH. The relative molecular mass of benzene is 78.0

A
  1. Calculate the relative empirical formula mass: EFr (CH) = 12.0 + 1.0 = 13.0
  2. The simplest whole number multiple relating the empirical and molecular formulae can be found by dividing the relative molecular mass by the relative empirical formula mass
  3. Mr/EFr = 78.0/13.0 = 6
  4. So the molecular formula is 6 x (CH) = C6H6
38
Q

what is a sig fig? what is it? what is not significant?

A

The number of sig figures indicates the precision of a measurement

A significant figure is an integer or a zero that follows an integer
Ex. 100.0 has 4 sig figs

A zero that comes before any integers is not significant
Ex. 0.0004 only has 1 sig fig

39
Q

what is the rule when finding significant figures with addition and subtraction?

A

The result of an addition or subtraction should have no more decimal places than the term with the least number of decimal places
Ex. 1.6 + 2.002 + 12 = 15.602 = 16 (two sig fig)

40
Q

what is the rule of sig figs when using multiplication and division?

A

The answer may not contain more significant figures than the factor with the least sig figs
Ex. 9.6 x 10.21 = 98

Rules for rounding off must be followed

41
Q

what are the rules of moving decimal places?

A

To the right the power on 10 is negative

To the left the power on 10 is positive

Chemistry (28 decks)

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