Mass Matters
Mass is never created or destroyed in a reaction, yet the reading on a balance can still seem to change. Learn conservation of mass, how to work out relative formula mass, and why an open system can appear to gain or lose mass.
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Mass never disappears
In any chemical reaction, mass is never created or destroyed. The atoms are just rearranged, so the total mass stays the same. Yet the reading on a balance can still seem to go up or down. This module explains why, and shows you how to work out the mass of a compound.
Conservation of mass words
Four terms run through this topic. Learn them first.
Open or sealed?
Whether gas can get in or out decides what the balance shows.
Match the term
- Conservation of mass
- Reactant
- Product
- Relative formula mass
- The total mass does not change in a reaction
- A substance you start with
- A substance the reaction makes
- The sum of the relative atomic masses in a formula
Mass in a reaction
In a chemical reaction, how does the total mass of the products compare with the total mass of the reactants?
- They are equal
- The products always weigh more
- The products always weigh less
- Some mass is destroyed
Why the balance seems to change
In an open system the balance can mislead you. If a gas escapes, such as carbon dioxide leaving when a carbonate reacts with acid, the mass seems to fall. If a gas is taken from the air, such as a metal gaining oxygen, the mass seems to rise. Mass is still conserved overall. Always name the gas that leaves or enters.
Match the measurement word
- Accuracy
- Precision
- Repeatability
- Reproducibility
- How close a result is to the true value
- How close repeated results are to each other
- The same person repeats it with the same method and equipment
- A different person or equipment gets the same result
Conservation of mass
Which TWO statements about conservation of mass are true? Choose all that apply.
- The total mass of reactants equals the total mass of products
- No atoms are created or destroyed
- Some mass is destroyed in the reaction
- Atoms can simply disappear
Work out the Mr
Calculate the relative formula mass, Mr, of water, H2O. Use the relative atomic masses Ar: H = 1 and O = 16.
How to find the Mr
Put the steps of working out a relative formula mass in order.
- Read the formula and list the elements
- Find the relative atomic mass of each element
- Multiply each Ar by how many atoms there are
- Add all the values together to get the Mr
Why mass is conserved
In a reaction, the total mass of the _____ equals the total mass of the products, because no atoms are created or _____. The relative formula mass is the sum of the relative _____ masses. In an open system, the mass can seem to fall if a _____ escapes. In a sealed system, the measured mass stays the _____.
Is mass really conserved?
Tap the TWO statements that are TRUE.
- In a sealed system, the measured mass stays the same.
- Mass is conserved in every chemical reaction.
- Atoms are destroyed during a reaction.
- The Mr of every substance is always 100.
Rise, fall or stay the same?
For each reaction, decide what happens to the mass shown on the balance.
- A carbonate reacts with acid in an open beaker, and carbon dioxide gas escapes. What happens to the measured mass?
- A metal is heated in an open dish and reacts with oxygen from the air. What happens to the measured mass?
- A reaction takes place inside a sealed, closed flask. What happens to the measured mass?
Explain the change
Explain why the mass on a balance can seem to change during a reaction in an open container, even though mass is always conserved.
- State the law of conservation of mass
- Explain what happens to the mass when a gas escapes in an open system
- Explain what happens when a gas is taken from the air, and why a sealed system shows no change