General Apparatus and Measurement Techniques

Measuring Liquid Volumes

You can use a standard plastic jug to measure milk, but it lacks the precision needed to measure exactly $25.00\text{ cm}^3$ of a strong acid.

Using a Measuring Cylinder For general-purpose measurements where high precision is not required:

1. Select a measuring cylinder with a capacity close to the volume you need (e.g., use a $10\text{ cm}^3$ cylinder for $8\text{ cm}^3$).
2. Pour liquid in until nearly at the mark, then use a teat pipette to reach the exact line.
3. Ensure the bottom of the meniscus is on the graduation line when viewed at eye level.

Using a Volumetric Pipette Used to deliver one fixed, highly accurate volume (e.g., $25.0\text{ cm}^3$):

1. Use a pipette filler to draw the liquid into the volumetric pipette until it is above the graduation mark.
2. Carefully release the liquid until the bottom of the meniscus sits exactly on the mark.
3. Allow the liquid to drain into the receiving flask under gravity.
4. Touch the tip of the pipette against the inside surface of the glass flask to ensure the final drop is delivered (pipettes are calibrated to account for the tiny volume remaining in the tip).

Using a Burette Used to measure variable volumes (titres) during a titration:

1. Rinse the burette with the solution and fill it, ensuring there are no air bubbles trapped in the tip below the tap.
2. Record the initial reading to two decimal places (ending in '0' or '5').
3. Run the liquid out into the reaction flask until the end point is reached.
4. Record the final reading to two decimal places.
5. Calculate the titre volume: $\text{Titre} = \text{Final Reading} - \text{Initial Reading}$.

Monitoring Gas Production

Understanding how to safely capture and measure an invisible gas explains why we can accurately track the speed of chemical reactions.

• A conical flask acts as the reaction vessel. It is easily swirled and sealed with an airtight rubber bung.
• A delivery tube passes through the bung and connects the flask to a gas syringe.
• As the chemical reaction produces gas, it travels through the delivery tube and pushes the graduated plunger of the syringe outwards.
• You record the volume of gas on the syringe scale at regular time intervals using a stopwatch.
• Gas syringes are highly advantageous because they do not require gas to be collected over water. This means water-soluble gases, like carbon dioxide, do not dissolve during collection.

The initial rate of a gas-producing reaction is calculated using:

$\text{Rate} = \frac{\text{Volume of gas collected}}{\text{Time taken}}$

Worked Example: Reaction Rate

In an experiment, $45.0\text{ cm}^3$ of carbon dioxide is collected in a gas syringe over $90\text{ s}$. Calculate the rate of reaction.

Step 1: Identify the values.

• $\text{Volume} = 45.0\text{ cm}^3$, $\text{Time} = 90\text{ s}$

Step 2: Substitute into the equation.

• $\text{Rate} = \frac{45.0}{90}$

Step 3: Calculate the final answer with units.

• $\text{Rate} = 0.50\text{ cm}^3\text{/s}$

Evaluating Measurements

Why do scientists calculate an average from multiple trials rather than just trusting their first result?

• Accuracy refers to how close a measured value is to the true, accepted value.
• Precision describes how tightly clustered a set of repeated results are. A high-resolution instrument does not automatically guarantee high precision if your experimental technique is inconsistent.
• To improve precision, you should repeat the experiment, discard anomalous results, and calculate a mean.
• Every measuring instrument has an inherent uncertainty. For analogue instruments, the uncertainty is generally plus or minus half the smallest graduation. For digital instruments, it is plus or minus the resolution itself.

The percentage uncertainty of a measurement is calculated using:

$\text{Percentage Uncertainty} = \left( \frac{\text{Absolute Uncertainty}}{\text{Measured Value}} \right) \times 100$

Worked Example: Burette Uncertainty

A student calculates a titre of $25.00\text{ cm}^3$. Each burette reading has an uncertainty of $\pm 0.05\text{ cm}^3$. Calculate the percentage uncertainty of the titre.

Step 1: Identify the values.

• Since a titre requires two separate readings (initial and final volume), the absolute uncertainty doubles: $2 \times 0.05 = 0.10\text{ cm}^3$.
• $\text{Measured Value} = 25.00\text{ cm}^3$

Step 2: Substitute into the equation.

• $\text{Percentage Uncertainty} = \left( \frac{0.10}{25.00} \right) \times 100$

Step 3: Calculate the final answer.

• $\text{Percentage Uncertainty} = 0.40\%$