Atomic Mass and Molecular Mass
When you fill a glass with tap water, how many atoms are in there? So many that there aren't enough numbers to picture them. An atom weighs so little that no kitchen or laboratory scale can detect it, so chemistry uses a unit made to fit its size.
Atomic mass: how much an atom weighs
If you try to weigh a speck of dust on a kitchen scale, it reads zero. But if you compare two tiny objects with each other, you can tell which one weighs twice as much. Atoms work the same way: they are compared with each other. So how do we assign a mass to each atom?
The lightest atom, hydrogen, serves as the reference: it weighs $1\ \text{amu}$. Carbon weighs $12\ \text{amu}$, which is $12$ times more; oxygen weighs $16\ \text{amu}$, $16$ times more. In the exercises the unit may appear written as u: it means the same thing.
What is the atomic mass of an element and in which unit is it measured? Look up the atomic mass of carbon and oxygen in the periodic table, and state which of the two…
What is the atomic mass of an element and in which unit is it measured? Look up the atomic mass of carbon and oxygen in the periodic table, and state which of the two atoms weighs more.
These are the atomic masses of four elements: $\text{H} = 1\ \text{u}$, $\text{C} = 12\ \text{u}$, $\text{N} = 14\ \text{u}$ and $\text{O} = 16\ \text{u}$. a) How many…
These are the atomic masses of four elements: $\text{H} = 1\ \text{u}$, $\text{C} = 12\ \text{u}$, $\text{N} = 14\ \text{u}$ and $\text{O} = 16\ \text{u}$.
- a) How many times heavier is a carbon atom than a hydrogen atom?
- b) And a nitrogen atom than a hydrogen atom?
Molecular mass: adding up atoms
If you snap three pieces of a building toy together, the whole block weighs the same as the sum of its three pieces. Does the same thing happen when several atoms bond to form a molecule?
The oxygen we breathe travels through the air forming molecules of $\text{O}2$. If the atomic mass of oxygen is $16\ \text{u}$, calculate the molecular mass of…
The oxygen we breathe travels through the air forming molecules of $\text{O}_2$. If the atomic mass of oxygen is $16\ \text{u}$, calculate the molecular mass of $\text{O}_2$.
The figure shows the model of a water molecule. Calculate the molecular mass of water, $\text{H}2\text{O}$ (atomic masses: $\text{H} = 1\ \text{u}$;…
The figure shows the model of a water molecule.
Calculate the molecular mass of water, $\text{H}_2\text{O}$ (atomic masses: $\text{H} = 1\ \text{u}$; $\text{O} = 16\ \text{u}$).
The gas that forms bubbles in soft drinks is carbon dioxide, $\text{CO}2$. The figure shows its molecule. Calculate its molecular mass (atomic masses:…
The gas that forms bubbles in soft drinks is carbon dioxide, $\text{CO}_2$. The figure shows its molecule.
Calculate its molecular mass (atomic masses: $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).
Explain in your own words the difference between atomic mass and molecular mass, and give an example of each.
Explain in your own words the difference between atomic mass and molecular mass, and give an example of each.
Calculating with subscripts and parentheses
An element's subscript tells you how many atoms of that element there are. When there is a set of parentheses with a subscript outside, that number multiplies everything inside.
The salt we add to food is $\text{NaCl}$. Calculate its molecular mass (atomic masses: $\text{Na} = 23\ \text{u}$; $\text{Cl} = 35.5\ \text{u}$).
The salt we add to food is $\text{NaCl}$. Calculate its molecular mass (atomic masses: $\text{Na} = 23\ \text{u}$; $\text{Cl} = 35.5\ \text{u}$).
Mollusc shells and school chalk are made of calcium carbonate, $\text{CaCO}3$. The figure shows the atoms that form it. Calculate its molecular mass (atomic masses:…
Mollusc shells and school chalk are made of calcium carbonate, $\text{CaCO}_3$. The figure shows the atoms that form it.
Calculate its molecular mass (atomic masses: $\text{Ca} = 40\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).
Glucose, $\text{C}6\text{H}{12}\text{O}6$, is the sugar that your muscles use when you do sport. Calculate its molecular mass (atomic masses: $\text{H} = 1\ \text{u}$;…
Glucose, $\text{C}_6\text{H}_{12}\text{O}_6$, is the sugar that your muscles use when you do sport. Calculate its molecular mass (atomic masses: $\text{H} = 1\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).
Sulphuric acid, $\text{H}2\text{SO}4$, is used in car batteries. Calculate its molecular mass (atomic masses: $\text{H} = 1\ \text{u}$; $\text{S} = 32\ \text{u}$;…
Sulphuric acid, $\text{H}_2\text{SO}_4$, is used in car batteries. Calculate its molecular mass (atomic masses: $\text{H} = 1\ \text{u}$; $\text{S} = 32\ \text{u}$; $\text{O} = 16\ \text{u}$).
Slaked lime, which is used in building work and vegetable gardens, has the formula $\text{Ca(OH)}2$. The figure shows its structure. Calculate its molecular mass…
Slaked lime, which is used in building work and vegetable gardens, has the formula $\text{Ca(OH)}_2$. The figure shows its structure.
Calculate its molecular mass carefully: the subscript $2$ outside the bracket multiplies everything inside it (atomic masses: $\text{Ca} = 40\ \text{u}$; $\text{O} = 16\ \text{u}$; $\text{H} = 1\ \text{u}$).
Comparing masses
You already know how to calculate molecular masses, so you can compare substances: how many times heavier one is than another, or which is the lightest in a group.
This also works for several molecules together: the mass of $n$ molecules is $n$ times the mass of one. And you can work backward too: if you know the molecular mass and all the atomic masses except one, the missing one is what's left after subtracting.
Nitrogen gas, $\text{N}2$, is used to fill bags of chips to keep them crunchy. Calculate its molecular mass (atomic mass: $\text{N} = 14\ \text{u}$).
Nitrogen gas, $\text{N}_2$, is used to fill bags of chips to keep them crunchy. Calculate its molecular mass (atomic mass: $\text{N} = 14\ \text{u}$).
The natural gas used in cookers is methane, $\text{CH}4$, and the ammonia in some cleaning products is $\text{NH}3$. The figure shows their molecules. a) Calculate the…
The natural gas used in cookers is methane, $\text{CH}_4$, and the ammonia in some cleaning products is $\text{NH}_3$. The figure shows their molecules.
- a) Calculate the molecular mass of each one.
- b) Which of the two molecules weighs more?
(Atomic masses: $\text{H} = 1\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{N} = 14\ \text{u}$.)
The molecular mass of $\text{CO}2$ is $44\ \text{u}$ and the atomic mass of oxygen is $16\ \text{u}$. Look at the model in the figure. Without looking at the periodic…
The molecular mass of $\text{CO}_2$ is $44\ \text{u}$ and the atomic mass of oxygen is $16\ \text{u}$. Look at the model in the figure.
Without looking at the periodic table, deduce the atomic mass of carbon and explain how you worked it out.
An oxygen molecule, $\text{O}2$, weighs $32\ \text{u}$ and a nitrogen molecule, $\text{N}2$, weighs $28\ \text{u}$. A hydrogen molecule, $\text{H}2$, weighs only…
An oxygen molecule, $\text{O}_2$, weighs $32\ \text{u}$ and a nitrogen molecule, $\text{N}_2$, weighs $28\ \text{u}$. A hydrogen molecule, $\text{H}_2$, weighs only $2\ \text{u}$.
- a) How many hydrogen molecules are needed together to weigh the same as one oxygen molecule?
- b) And to weigh the same as one nitrogen molecule?
Order these molecules from lowest to highest molecular mass, calculating the mass of each one first: $\text{SO}2$, $\text{O}2$, $\text{CO}2$ and $\text{CH}4$ (atomic…
Order these molecules from lowest to highest molecular mass, calculating the mass of each one first: $\text{SO}_2$, $\text{O}_2$, $\text{CO}_2$ and $\text{CH}_4$ (atomic masses: $\text{H} = 1\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$; $\text{S} = 32\ \text{u}$).
Mass percent and average masses
What part of a molecule's mass belongs to each element? It is calculated with a percent.
Two more ideas for the hard exercises:
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Atomic mass on the table with decimals. Chlorine has atoms of $35\ \text{amu}$ and others of $37\ \text{amu}$ (isotopes). The value on the table, $35.5\ \text{amu}$, is an average based on how abundant each type is: it ends up closer to the more abundant one.
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Weighted average mass. If a mixture has $80\ \%$ of a molecule of $28\ \text{amu}$ and $20\ \%$ of one of $32\ \text{amu}$, the average is $0.80 \times 28 + 0.20 \times 32$. If a molecule weighs more than that average, it is heavier than the "mean" of the mixture.
The molecular mass of calcium carbonate, $\text{CaCO}3$, is $100\ \text{u}$. The bar in the figure shows how that mass is divided. a) Calculate what percentage of the…
The molecular mass of calcium carbonate, $\text{CaCO}_3$, is $100\ \text{u}$. The bar in the figure shows how that mass is divided.
- a) Calculate what percentage of the mass corresponds to calcium ($40\ \text{u}$).
- b) Do the same for carbon ($12\ \text{u}$) and for oxygen (three atoms of $16\ \text{u}$ in total).
- c) Check that the three percentages add up to $100\ \%$.
Sulfur dioxide, $\text{SO}2$, is released by volcanoes and is also used as a preservative in dried fruit. a) Calculate its molecular mass ($\text{S} = 32\ \text{u}$;…
Sulfur dioxide, $\text{SO}_2$, is released by volcanoes and is also used as a preservative in dried fruit.
- a) Calculate its molecular mass ($\text{S} = 32\ \text{u}$; $\text{O} = 16\ \text{u}$).
- b) What percentage of the mass of the molecule is sulfur? And oxygen?
- c) Explain why the two percentages must add up to exactly $100\ \%$.
The atomic mass of chlorine shown in the periodic table is $35.5\ \text{u}$, but no chlorine atom weighs exactly that: some chlorine atoms weigh $35\ \text{u}$ and…
The atomic mass of chlorine shown in the periodic table is $35.5\ \text{u}$, but no chlorine atom weighs exactly that: some chlorine atoms weigh $35\ \text{u}$ and others weigh $37\ \text{u}$ (these are called isotopes). The figure represents the two types of atoms.
- a) Explain in your own words how it is possible for the atomic mass in the periodic table to be $35.5\ \text{u}$.
- b) Reason which of the two types of chlorine atoms will be more abundant in nature.
From data to substance: an unknown element and the name of the salt
If you know the molecular mass of a compound and the masses of all its atoms except one, you can find the missing one and, using the periodic table, identify the element.
To name a simple salt of a metal with chlorine, you write the metal's name followed by "chloride": $\text{NaCl}$ is sodium chloride, $\text{KCl}$ is potassium chloride, and $\text{MgCl}_2$ is magnesium chloride. If the metal can have several charges, a Roman numeral is added in parentheses, as in iron(III) chloride. Complete nomenclature is covered in another section.
A salt formed by a metal and chlorine has the formula $\text{XCl}2$ and a molecular mass of $111\ \text{u}$. The figure shows its model. a) Knowing that…
A salt formed by a metal and chlorine has the formula $\text{XCl}_2$ and a molecular mass of $111\ \text{u}$. The figure shows its model.
- a) Knowing that $\text{Cl} = 35.5\ \text{u}$, calculate the atomic mass of element $\text{X}$.
- b) Look it up in the periodic table and identify the element.
- c) Write the full name of the salt.
A glucose molecule, $\text{C}6\text{H}{12}\text{O}6$, has a molecular mass of $180\ \text{u}$. a) How many water molecules ($18\ \text{u}$) weigh the same as one glucose…
A glucose molecule, $\text{C}_6\text{H}_{12}\text{O}_6$, has a molecular mass of $180\ \text{u}$.
- a) How many water molecules ($18\ \text{u}$) weigh the same as one glucose molecule?
- b) How many hydrogen molecules ($2\ \text{u}$)?
- c) Do the same calculation for oxygen ($32\ \text{u}$). Do you get an integer? Explain what it means if it does not come out exact.
Air is approximately $80\ \%$ nitrogen $\text{N}2$ ($28\ \text{u}$) and $20\ \%$ oxygen $\text{O}2$ ($32\ \text{u}$). The figure represents a sample of air. a) Calculate…
Air is approximately $80\ \%$ nitrogen $\text{N}_2$ ($28\ \text{u}$) and $20\ \%$ oxygen $\text{O}_2$ ($32\ \text{u}$). The figure represents a sample of air.
- a) Calculate the average molecular mass of air. Hint: weight each mass according to its percentage.
- b) Does a molecule of $\text{CO}_2$ ($44\ \text{u}$) weigh more or less than an "average" molecule of air?
- c) Use the result to reason why, in a closed room without draughts where $\text{CO}_2$ is being released (for example, a cellar where grape must is fermenting), this gas tends to accumulate near the floor.
Compare without a balance, using only calculations: a) Which weighs more: two water molecules ($18\ \text{u}$ each) or one oxygen molecule ($32\ \text{u}$)? b) What…
Compare without a balance, using only calculations:
- a) Which weighs more: two water molecules ($18\ \text{u}$ each) or one oxygen molecule ($32\ \text{u}$)?
- b) What about three nitrogen molecules ($28\ \text{u}$ each) or two of carbon dioxide ($44\ \text{u}$ each)?
- c) Explain the procedure you followed to compare them.
Two closed bags each contain $100$ molecules: the first contains methane, $\text{CH}4$, and the second sulfur dioxide, $\text{SO}2$ (atomic masses:…
Two closed bags each contain $100$ molecules: the first contains methane, $\text{CH}_4$, and the second sulfur dioxide, $\text{SO}_2$ (atomic masses: $\text{H} = 1\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$; $\text{S} = 32\ \text{u}$).
- a) Calculate the molecular mass of each gas.
- b) How much does the contents of each bag weigh, in $\text{u}$?
- c) How many times more does one bag weigh than the other? Justify your answer.
Back to the beginning: we don't count atoms one by one or weigh them on a scale; we compare them with the amu and add up atomic masses to find the mass of any molecule.