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You have a sample that contains 5 moles of zinc.

  1. What is the mass of the zinc in the sample?
  2. How many atoms of zinc are in the sample?
  1. Molar mass of zinc is 65.38 g.mol - 1 , meaning that 1 mole of zinc has a mass of 65.38 g.

  2. If 1 mole of zinc has a mass of 65.38 g, then 5 moles of zinc has a mass of:

    65.38 g x 5 mol = 326.9 g (answer to a)

  3. 5 × 6 . 023 × 10 23 = 30 . 115 × 10 23

    (answer to b)

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Moles and molar mass

  1. Give the molar mass of each of the following elements:
    1. hydrogen
    2. nitrogen
    3. bromine
  2. Calculate the number of moles in each of the following samples:
    1. 21.62 g of boron (B)
    2. 54.94 g of manganese (Mn)
    3. 100.3 g of mercury (Hg)
    4. 50 g of barium (Ba)
    5. 40 g of lead (Pb)

An equation to calculate moles and mass in chemical reactions

The calculations that have been used so far, can be made much simpler by using the following equation:

n ( number of moles ) = m ( mass of substance in g ) M ( molar mass of substance in g · mol - 1 )
Remember that when you use the equation n = m M , the mass is always in grams (g) and molar mass is in grams per mol ( g · mol - 1 ).

The equation can also be used to calculate mass and molar mass, using the following equations:

m = n × M


M = m n

The following diagram may help to remember the relationship between these three variables. You need to imagine that the horizontal line is like a 'division' sign and that the vertical line is like a 'multiplication' sign. So, for example, if you want to calculate 'M', then the remaining two letters in the triangle are 'm' and 'n' and 'm' is above 'n' with a division sign between them. In your calculation then, 'm' will be the numerator and 'n' will be the denominator.

Calculate the number of moles of copper there are in a sample that weighs 127 g.

  1. n = m M
  2. n = 127 63 . 55 = 2

    There are 2 moles of copper in the sample.

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You are given a 5 mol sample of sodium. What mass of sodium is in the sample?

  1. m = n × M
  2. M N a = 22.99 g.mol - 1


    m = 5 × 22 . 99 = 114 . 95 g

    The sample of sodium has a mass of 114.95 g.

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Calculate the number of atoms there are in a sample of aluminium that weighs 80.94 g.

  1. n = m M = 80 . 94 26 . 98 = 3 m o l e s
  2. Number of atoms in 3 mol aluminium = 3 × 6.023 × 10 23

    There are 18.069 × 10 23 aluminium atoms in a sample of 80.94 g.

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Some simple calculations

  1. Calculate the number of moles in each of the following samples:
    1. 5.6 g of calcium
    2. 0.02 g of manganese
    3. 40 g of aluminium
  2. A lead sinker has a mass of 5 g.
    1. Calculate the number of moles of lead the sinker contains.
    2. How many lead atoms are in the sinker?
  3. Calculate the mass of each of the following samples:
    1. 2.5 mol magnesium
    2. 12 mol lithium
    3. 4.5 × 10 25 atoms of silica

Molecules and compounds

So far, we have only discussed moles, mass and molar mass in relation to elements . But what happens if we are dealing with a molecule or some other chemical compound? Do the same concepts and rules apply? The answer is 'yes'. However, you need to remember that all your calculations will apply to the whole molecule . So, when you calculate the molar mass of a molecule, you will need to add the molar mass of each atom in that compound. Also, the number of moles will also apply to the whole molecule. For example, if you have one mole of nitric acid (HNO 3 ), it means you have 6.023 x 10 23 molecules of nitric acid in the sample. This also means that there are 6.023 × 10 23 atoms of hydrogen, 6.023 × 10 23 atoms of nitrogen and (3 × 6.023 × 10 23 ) atoms of oxygen in the sample.

Questions & Answers

how to calculate forces of different direction?
Lerato Reply
What is atoms
siyanela Reply
how to draw a free body diagram
Mickey Reply
When calculating acceleration in a ticket timer experiment, which time do we use?
Sarah Reply
The time elapsed between the dots you are using. That will be the amount of intervals × the period of the ticker timer.
when the planet Jupiter is closest to earth it is 6.28×10^8 km away.if Jupiter has a mass of 1.9×10^27 kg, what is the magnitude of the gravitational force between Jupiter and Earth?
zandi Reply
can I join but I'm in grade ten
how to calculate the speed wave
Moses Reply
how do we calculate acceleration with an angle
Nhlamulo Reply
draw separate free body diagrams for each blocks
Mudau Reply
VC become the same as the HC
Lidya Reply
determine all the forces acting on an object
Angle Reply
what is the shape of the NH3
Mathabathi Reply
Trigonal planner
please can you show me Newtons law and Application of Newtons law
Lutho Reply
meaning of frictional force
Thato Reply
Fictional force is a force that opposes motion and act parallel
...to the surface
how do you calculate static force
The force of static can be calculated by taking the coefficient of friction between the two surfaces and multiplying it by the normal force that the surface is applying to the object. On a flat surface usually the normal force is equal to the force of gravity acting down on the object
what is a vector
Gladwin Reply
it is a quantity with both magnitude and direction
is the physical quantity with both magnitude and direction
it is true tomorrow we are writting pysics?
not sure
we are writing physical science tomorrow
What grade are you guys?
i am in grade 11
what is Newton's first law of motion
Amkela Reply
an object will remain in a state of rest or continue traveling at constant velocity unless acted upon by an unbalanced (net) force
what is Newton second law and third law
NSL-when a net force acts on an object,the object wil accelerate in the direction of the force at an acceleration directly proportional to the force and inversly proportional to the mass of the object

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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