Measurement, Mass & Equations Flashcards

All 28 cards in this deck

State the law of conservation of mass.

No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.

Why must a symbol equation be balanced?

Because no atoms are lost or made, so there must be the same number of atoms of each element on both sides.

True or false? When balancing an equation you may change a subscript inside a formula to make the atoms balance.

False. Only the large multipliers in front of formulae may be changed; changing a subscript changes the substance.

What are the steps to balance a symbol equation?

e.g. $\mathrm{Ba} + \mathrm{HCl} \rightarrow \mathrm{BaCl_2} + \mathrm{H_2}$

1. Count atoms of each element on both sides: Cl is 1 on the left, 2 on the right.
2. Put a large multiplier in front of a whole formula: $2\mathrm{HCl}$.
3. Re-count until every element matches: $\mathrm{Ba} + 2\mathrm{HCl} \rightarrow \mathrm{BaCl_2} + \mathrm{H_2}$

How many atoms of each element are shown by a normal-script multiplier in front of a formula?

e.g. $3\mathrm{H_2O}$

The multiplier multiplies every atom in the formula — $3\mathrm{H_2O}$ is 6 hydrogen atoms and 3 oxygen atoms.

True or false? In a sealed (closed) container, the total mass does not change during a chemical reaction.

True. No atoms are lost or made and nothing can escape, so the mass stays the same.

What is the relative formula mass ($M_r$) of a compound?

The sum of the relative atomic masses ($A_r$) of all the atoms in the numbers shown in the formula.

What are the steps to calculate the relative formula mass ($M_r$) of a compound?

e.g. $\mathrm{H_2O}$ ($A_r$: H = 1, O = 16)

1. Count the atoms of each element: 2 H and 1 O.
2. Multiply each $A_r$ by its number of atoms: $2 \times 1 = 2$; $1 \times 16 = 16$.
3. Add them: $M_r = 18$

In a balanced equation, how does the total $M_r$ of the reactants compare with the total $M_r$ of the products?

They are equal, when each $M_r$ is counted in the quantities shown by the multipliers.

How do you deduce a missing $A_r$ or $M_r$ from a balanced equation?

e.g. $3\mathrm{H_2} + \mathrm{RO_3} \rightarrow \mathrm{R} + 3\mathrm{H_2O}$, reactants total 150

1. Total $M_r$ of products = total $M_r$ of reactants = 150.
2. Work out the known products: $3 \times M_r(\mathrm{H_2O}) = 3 \times 18 = 54$.
3. Subtract from the total: $A_r(\mathrm{R}) = 150 - 54 = 96$

What is the formula for the percentage by mass of an element in a compound?

$\%$ by mass $= \dfrac{A_r \times \text{number of atoms of that element}}{M_r \text{ of the compound}} \times 100$

What are the steps to calculate the percentage by mass of an element in a compound?

e.g. Cl in $\mathrm{CH_3Cl}$ ($A_r$: H = 1, C = 12, Cl = 35.5)

1. Find the $M_r$ of the compound: $12 + 3 + 35.5 = 50.5$.
2. Find the total mass of that element: $1 \times 35.5 = 35.5$.
3. Divide and $\times 100$: $\dfrac{35.5}{50.5} \times 100 = 70.3\%$

True or false? Relative formula mass ($M_r$) has the unit grams.

False. $M_r$ is a relative value and has no units.

Why does the mass of the solid increase when a metal is heated in air?

Oxygen atoms from the air join the metal to form the oxide, so the oxide contains the mass of the metal plus the mass of the oxygen.

Why does the mass in an open flask decrease when a metal carbonate is thermally decomposed?

Carbon dioxide gas is produced and escapes into the atmosphere, leaving only the metal oxide as solid.

True or false? An apparent loss of mass in an open flask shows that atoms have been destroyed.

False. A gaseous product has escaped from the flask; no atoms are lost or made.

What happens to the mass of an open flask when an acid reacts with a carbonate, and why?

The mass decreases, because carbon dioxide is produced and escapes from the flask.

In terms of particles, why can a reaction in a non-enclosed system appear to gain mass?

Particles of a gaseous reactant (e.g. oxygen) from the air are added to the solid, and their mass was not counted at the start.

True or false? If magnesium is burned in a sealed container, the total mass stays the same.

True. No gas can enter or escape, so the total mass of reactants equals the total mass of products.

How do you use a balanced equation to predict whether mass rises or falls in an open container?

e.g. $\mathrm{CaCO_3} \rightarrow \mathrm{CaO} + \mathrm{CO_2}$

1. Find any gases in the equation: $\mathrm{CO_2}$.
2. Decide if the gas is a reactant (enters) or product (escapes): it is a product.
3. Gas escaping means mass falls: the mass in the container decreases.

Why is there always some uncertainty in a measurement?

Every measuring instrument and every reading has limited precision, so the true value is never known exactly.

What is the range of a set of repeat measurements?

The difference between the largest and smallest value.

How is the range of repeat measurements used as a measure of uncertainty?

The uncertainty is quoted as $\pm$ half the range about the mean.

What are the steps to find the mean and uncertainty of repeat measurements?

e.g. $37.6$, $37.2$, $37.8$, $37.4\,^\circ$C

1. Add the values and divide by how many: mean $= 37.5\,^\circ$C.
2. Range $= 37.8 - 37.2 = 0.6\,^\circ$C.
3. Uncertainty $= \pm$ half the range: $37.5 \pm 0.3\,^\circ$C

What does the $\pm 0.3\,^\circ$C mean in a result written as $16.5 \pm 0.3\,^\circ$C?

It is the uncertainty — the values lie up to $0.3\,^\circ$C either side of the mean.

What is an anomalous result?

A result that does not fit the pattern of the others and is much larger or smaller than expected.

True or false? Anomalous results should be included when calculating the mean.

False. Anomalous results should be identified and excluded before calculating the mean.

True or false? A smaller range in a set of repeat measurements means a smaller uncertainty.

True. The results are closer together, so the uncertainty about the mean is smaller.