Atomic mass and molecular mass
An atom weighs far less than you can imagine: you would need to put billions and billions of them together before a balance could notice anything. That is why, in chemistry, atoms are not weighed in grams, but with a unit made to their size.
Atomic mass: how much does an atom weigh?
If you try to weigh a speck of dust on a kitchen scale, it reads zero because it is far too light, but if you compare two tiny objects with each other you can tell which one weighs twice as much: how do we assign a mass to atoms if there is no balance capable of weighing them one at a time?
The lightest atom, hydrogen, was used as the reference: it weighs $1\ \text{u}$. Carbon weighs $12\ \text{u}$, that is, $12$ times more; oxygen weighs $16\ \text{u}$, $16$ times more.
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 join three building pieces together to make a block, the whole block weighs exactly the same as its three pieces added up separately: does the same 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: $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).
Calculating with subscripts and brackets
An element's subscript tells you how many atoms of that element there are. When there are brackets 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: $\text{Ca} = 40\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).
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
Once you know how to calculate molecular masses, you can compare substances with each other: how many times heavier one is than another, or which is the lightest in a group.
Nitrogen gas, $\text{N}2$, is used to fill bags of crisps 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 crisps to keep them crunchy. Calculate its molecular mass (atomic mass: $\text{N} = 14\ \text{u}$).
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: $\text{H} = 1\ \text{u}$; $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$; $\text{S} = 32\ \text{u}$).