Molecular Mass: Solved Questions on Relative Molecular Mass

len Alfred Ajibola - Tue, 12th February, 2019 @ 11:12: AM

Topics in Chemistry

Chemistry Scheme of Work, SS1, First Term Chemistry Scheme of Work, SS1, Second Term Chemistry Scheme of Work, SS1, Third Term Compounds in Chemistry: Characteristics of Compounds Types of Mixture: Homogenous and Heterogeneous Mixtures Mixtures: Characteristics of Mixtures Physical Properties of Matter explained with Examples Chemical Properties of Matter explained with Examples Differences between Physical and Chemical Change Ionic Theory and Electrolysis Physical and Chemical Properties of Acids States of Matter: Characteristics of Solids Liquid State of Matter: Characteristics of Liquids Gaseous State of Matter: Characteristics of Gases Molar Mass, Worked Examples and Differences with Molecular Mass Elements: Characteristics of Elements Salts and Types of Salt in Chemistry Mixtures and Separation of Mixtures Mixtures and Separation of Mixtures - part 2 Mixtures and Separation of Mixtures - part 3

Academic Questions in Chemistry

Please check out our Test Your Knowledge page to see all Questions and Answers

The Separating Funnel is a technique used in the separation of mixtures. Another name for Separating Funnel is the _____ Funnel.

  • A. Partition
  • B. Separatory
  • C. Division
  • D. Liquid
  • E. Half
  • F. Dividing

2NH4+(aq) + Ag+(aq) -> 2H4AgN

The equation above represents _____ acid.

  • A. Svante Arrhenius'

  • B. Brønsted–Lowry's

  • C. Gilbert Lewis'

  • D. Avogadro's

  • E. Sir Isaac Newton's

  • F. Albert Einstein's

According to Brønsted Thomas Martin Lowry, an acid is defined as _____.

  • A. Any substance that produces hydrogen ion (H+) when dissolved in water

  • B. A substance that accepts electron pair in a chemical reaction

  • C. A substance that gives out hydronium ion (H3O+) when dissolved in water

  • D. A substance that donates a proton in a chemical reaction

  • E. A substance that reacts with a base to form salt and water

  • F. A substance whose hydrogen ions  (H+) are displaced after a reaction with metals higher in the activity, reactivity or electrochemical series

Which of the following is NOT a characteristic of chemical change?

  • A. A new thing is formed
  • B. It cannot be easily separated into it's constituents elements
  • C. A new chemical formula accompanies whatever is formed
  • D. The burning of candle is an example of a chemical change
  • E. Blood is an example of a chemical change
  • F. Iron rust is an example of a chemical change

Which of the following statement is FALSE concerning a physical change _____.

  • A. Substances formed in a physical change have a unique chemical formula and IUPAC name
  • B. It's a change that is easily reversible
  • C. It's a change that can be separated via an appropriate separation technique
  • D. No new substance is formed
  • E. A mixture of salt and water is a physical change
  • F. Physical change can result to a change in color and shapes of substances

The following substances will sublime EXCEPT _____.

  • A. Sodium Cryolite
  • B. Solid Carbon(IV)oxide
  • C. Arsenic
  • D. Sulphur
  • E. Iodine Crystals
  • F. Naphthalene

Which of the following is a state of matter?

  • A. Ions
  • B. Plasma
  • C. Compounds
  • D. Molecules
  • E. Atoms
  • F. None of the above

The ideal way of separating a mixture of blood is through _____.

  • A. Chromatography
  • B. Fractional Distillation
  • C. Centrifugation
  • D. Filtration
  • E. Separating Funnel
  • F. Fractional Crystallization



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What is Molecular Mass?

The amount or quantity of a compound (or molecule) in chemistry can be measured in a variety of ways. The measured quantities will include:

  1. Mole of a substance.
  2. Molecular Mass of a compound.
  3. Molar Mass of a compound and so on.
  4. Please read on Molar Mass, Worked Examples and it's Differences with Molecular Mass here.

In this article, we will explore the concepts of molecular mass.

Firstly, the term molecule refers to the composition and arrangement of atoms expressed in ratios.

Note: The atoms forming a molecule can be similar or different. For instance:

  • O2 is oxygen molecule (where the oxygen atoms are combined in a ratio of 1:1).
  • In ozone molecule, that is O3, The oxygen atoms are combined in a ratio of 1:1:1.
  • Water (H2O) is a molecule composed of hydrogen and oxygen in the ratio 2:1.
  • Please read on Emperical Formula here.

Recall that all elements have got their respective atomic masses; for instance Hydrogen (H) has an atomic mass of 1.00794 while that of carbon (C) is 12.011. The atomic mass of all the 108 elements can be seen on the periodic table.

Please read on the Periodic Table here.

The molecular mass of a substance or molecule is usually defined in relation to 12C (Carbon-12).

This is why it’s preferably called Relative Molecular Mass (RMM).

The Relative Molecular Mass value is obtained from the sum of the atomic masses of the constituent atoms or elements that make up the molecule.

Its unit is the Atomic Mass Unit (amu) or Dalton (Da).

Please read on Elements and their Characteristics here.

Note: To calculate the Relative Molecular Mass, we will have to take into consideration the atomic masses of each atom present in the molecule.


Solved Questions on Molecular Mass

Example 1

Calculate the Relative Molecular Mass of Methane (CH4). (Carbon=12.001 and H=1.00794)

  • Molecular Mass = Atomic mass of carbon + Atomic mass of Hydrogen
  • Methane is composed of one atom of Carbon and 4 atoms of Hydrogen.
  • 12.001 + (1.00794 x 4)

Molecular Mass of Methane = 16.033 amu

You can Attempt LEN ACADEMY Questions and Answers on various Subjects here.


Example 2

Calculate the Relative Molecular Mass of Water (H2O). (Oxygen=15.9994 and Hydrogen=1.00794)

  • Molecular Mass = Atomic mass of Hydrogen + Atomic mass of Oxygen
  • Water contains 2 atoms of Hydrogen and 1 atom of oxygen.
  • (1.00794 x 2) + 15.9994

Molecular Mass of Water = 18.015 Da

Please read on Emperical Formula and its Calculations here.


Example 3

Calculate the Relative Molecular Mass of Ethanol (C2H5OH). (Carbon=12.001, Oxygen = 15.9994 and Hydrogen = 1.00794)

  • Molecular Mass = Atomic mass of Carbon + Atomic mass of Hydrogen + Atomic mass of Oxygen
  • Ethanol contains 2 atoms of Carbon, 6 atoms of Hydrogen and 1 atom of oxygen.
  • (12.001 x 2) + (1.00794 x 5) + 15.9994

Molecular Mass of Ethanol = 45.036 amu

Please read on Graham's Law of Diffusion with some worked Examples here.

Note: We used 3 significant digits in all our answers.

This is so because the relative molecular mass is always defined and expressed in terms of 12C (Carbon-12) whose atomic mass has 3 significant digits.

The atomic mass of 12C is 12.011

A major challenge encountered when calculating molecular mass is that it becomes difficult or impossible to calculate especially when the relative molecular mass of large molecules, polymers and macromolecules are involved.

Examples of large molecules (with indefinite molecular masses) include carbohydrates, cellulose and complex sugars.

The large molecules (above) have no specific chemical formula throughout their volume.

Please read more on Carbohydrates and Sugars here.

Note: Relative Molecular Mass prove to be useful only when we calculate substances with small and definite molecular sizes. This have been proven through the Modifications of Dalton's Atomic Theory.

Please read on Dalton's Atomic Theory and its Modifications here.

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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Amazing facts in Chemistry

The only letters that failed to appear on the periodic table are letters:

J     &     Q

Gold and copper are the only two non-silvery colored metals.

Please read some interesting facts about gold here

Copper is the only metal that is naturally antibacterial. This is why you would see some children utilize 'copper' water bottles in schools

Water actually freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy.

Please read on modification of Dalton's atomic theory here.

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre