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Molar Mass (DP IB Chemistry): Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Philippa Platt

Updated on

DP Chemistry: SLDP Chemistry: HL

Molar Mass

  • Atoms are incredibly small, so any measurable amount of a substance must contain a vast number of atoms

  • These large numbers are difficult to work with, so we use moles as a more practical unit for measuring quantities of substances

  • When calculating the number of particles in a substance, we usually refer to the number of moles

  • The number of moles or particles can be calculated easily using a formula triangle

Formula triangle diagram linking moles, particles and Avogadro's constant 

Triangle diagram showing relationship between particles, moles, and Avogadro's constant labelled L. Includes division and multiplication symbols.
The moles and particles formula triangle – cover with your finger the one you want to find out and follow the directions in the triangle

Worked Example

How many hydrogen atoms are in 0.010 moles of CH3CHO?

Answer:

  • There are 4 H atoms in 1 molecule of CH3CHO

  • So, there are 0.040 moles of H atoms in 0.010 moles of CH3CHO

number of H atoms = amount in moles x L

0.040 moles x (6.02 x 1023) = 2.4 x 1022 atoms

Worked Example

How many moles of hydrogen atoms are in 3.612 x 1023 molecules of H2O2?

Answer:

  • In 3.612 x 1023 molecules of H2O2  there are 2 x (3.612 x 1023) atoms of H

  • So, there are 7.224 x 1023 atoms of H

number of moles of H atoms = fraction numerator bold number bold space bold of bold space bold particles over denominator bold italic L end fraction

fraction numerator bold 7 bold. bold 224 bold space bold cross times bold 10 to the power of bold 23 over denominator bold 6 bold. bold 02 bold space bold cross times bold 10 to the power of bold 23 end fraction= 1.20 moles of H atoms

Moles and Mass

  • We measure moles by weighing the mass of a substance

  • The number of moles can be found using a formula triangle

  • The molar mass of a substance is its relative atomic mass (Ar) or relative formula mass (Mr) expressed in grams

  • Molar mass is given in units of g mol⁻¹

Formula triangle diagram linking moles, mass and molar mass 

Triangle diagram for mass, moles, and molar mass with equations: mass = moles × M, mass/molar mass = moles, molar mass in g/mol.
The moles and mass formula triangle – cover with your finger the one you want to find out and follow the directions in the triangle

Worked Example

What is the mass of 0.250 moles of zinc?

Answer:

  • From the periodic table the relative atomic mass of Zn is 65.38

  • So, the molar mass is 65.38 g mol-1

  • The mass is calculated by moles x molar mass

  • This comes to 0.250 mol x 65.38 g mol-1 = 16.3 g

Worked Example

How many moles are in 2.64 g of sucrose, C12H11O22  (Mr = 342.3)?

Answer:

  • The molar mass of sucrose is 342.3 g mol-1

  • The number of moles is found by 

moles = fraction numerator bold mass bold space stretchy left parenthesis g stretchy right parenthesis over denominator bold molar bold space bold mass bold space stretchy left parenthesis g space mol to the power of negative 1 end exponent stretchy right parenthesis end fraction

  • This comes to 

moles = fraction numerator bold 2 bold. bold 64 bold space bold g over denominator bold 342 bold. bold 3 bold space bold g bold space bold mol to the power of bold minus bold 1 end exponent end fraction =  7.71 x 10-3 mol

Examiner Tips and Tricks

Always show your workings in calculations as its easier to check for errors and you may pick up credit if you get the final answer wrong.

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    Author
    Reviewer
    Alexandra Brennan

    Author: Alexandra Brennan

    Expertise: Chemistry Content Creator

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener