Introduction to Lab Equipment

The metric system, which is used in science worldwide, is a decimal-based system of measurement. Common units for volume include:

• Volume: liter (L) and milliliter (mL)
• Mass: gram (g) and milligram (mg)

Different instruments vary in how well they measure volume.

• Beaker: Cylindrical container with markings for rough estimates. Not designed for precise measurements — typically ±5% uncertainty. Best for mixing or holding liquids.

• Erlenmeyer flask: Cone-shaped with a narrow neck, good for swirling without spilling. Like a beaker, its markings are approximate.

• Graduated cylinder: Tall and narrow with marked scale. Designed for accurate volume measurements.

  • Smaller cylinders generally have finer scale markings, so they can be read more precisely.

• Pipette or burette (not used in this lab): Tools for very precise delivery of small volumes.

To correctly read a graduated cylinder, always place it on a flat surface and bring your eye level to the surface of the liquid to avoid parallax error, which occurs when the reading changes depending on the viewing angle. Most liquids curve in the cylinder; this curve is called the meniscus. For water and most liquids, you should read from the bottom of the meniscus and record your measurement to one decimal place beyond the smallest marking. Using a funnel when transferring liquids into narrow graduated cylinders can help prevent spills and ensure you transfer the entire sample, which is critical for accurate measurements.

Mass can be measured with a top-loading balance, which reads to the nearest 0.01 g and is suitable for general use, or with an analytical balance, which reads to the nearest 0.0001 g and must be handled carefully to avoid errors caused by breezes, moisture, or unstable placement. We will use an analytical balance in this course.

Accuracy describes how close your measurement is to the true or accepted value, while precision describes how close repeated measurements are to one another, regardless of whether they are accurate. For example,

• • Example: If you weigh a 100.00 g sample and your balance reads 99.98 g, that’s very accurate.

  • Example: If you weigh the same object five times and always get 98.76 g, 98.77 g, 98.75 g, your results are precise (but not accurate if the true mass is 100.00 g)

Accuracy is generally determined by percent error. The formula for percent error is:

$$\text{Percent Error} = \frac{|\text{Measured Value} – \text{True Value}|}{\text{True Value}} \times 100\%$$

Precision is generally determined by the sensitivity of the instrument/tool (how many tick marks does the ruler have?) and by the skill of the person using the instrument. The precision of an instrument depends on the smallest marked division on its scale. A good rule for recording measurements is to write down all certain digits and then estimate one more digit. For example, if a graduated cylinder has markings every 0.1 mL, you can estimate between the marks and record your measurement to the nearest 0.01 mL.

To determine the precision of a data set with multiple trials, standard deviation is generally used. The formula for standard deviation is:

$$s=\sqrt{\frac{\sum (x_i-\bar{x}{)}^2}{n-\mathrm{1<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif"></span>}}}$$

Precision and accuracy in measurements are not just important in the laboratory — they matter in everyday life. A pharmacist must measure liquid medications accurately to ensure correct dosages for patients, and a baker scaling ingredients for bread needs accurate mass measurements to produce consistent results. Environmental scientists depend on precise measurements to detect small but important changes in water quality over time. In this lab, you will see firsthand how the choice of measurement tool, proper technique, and careful reading can make a measurable difference in your results.

$$s = \sqrt{ \frac{1}{n – 1} \sum_{i=1}^{n} (x_i – \bar{x})^2 }$$