Target Practice

3. Calculate the Percent Error for Team Beta's average measurement. Show formula setup and final value:

4. Compare Team Beta and Team Gamma. Which team would you trust more to produce reliable lab results if their equipment were recalibrated? Justify your reasoning.

3 Part 3: Error Classification (Systematic vs. Random)

Case A: The Dirty Pipette

A student repeatedly reads a meniscus from an awkward angle (parallax error), causing every single delivered volume to be 0.4 mL lower than intended.

Error Type:

Affects: (Acc / Prec)

Case B: Air Current Drafts

An electronic balance shield is left open near an AC vent. The digital numbers constantly flutter up and down unpredictably across trials.

Error Type:

Affects: (Acc / Prec)

Rapid Exit Check (Tear or Hand In)

30-Min Wrap-Up

Q1: Can a set of measurements be precise without being accurate? Provide one concise real-world chemistry laboratory example.

Q2: A chemist measures the boiling point of pure water 4 times: 104.1°C, 104.2°C, 104.0°C, 104.1°C (accepted: 100.0°C). Characterize the student's data:

High Accuracy, High Precision Low Accuracy, High Precision High Accuracy, Low Precision Low Accuracy, Low Precision

Target Practice: Accuracy vs. Precision — Chemistry Worksheet Page 2 of 2

Accepted Density = 8.96 g/cm³

Question 3: Percent Error Calculation for Team Beta:

\(\text{Percent Error} = \frac{|\text{Experimental} - \text{Accepted}|}{\text{Accepted}} \times 100\%\)

\(\text{Percent Error} = \frac{|7.21 - 8.96|}{8.96} \times 100\% = \frac{1.75}{8.96} \times 100\% \approx \mathbf{19.53\%}\)

Accept 19.5% or 20% depending on significant figure rules enforced in class.

Question 4: Team Beta vs. Team Gamma — Which team to trust?

Trust Team Beta. Team Beta demonstrated high precision (range of only 0.02 g/cm³), proving their laboratory technique is consistent, careful, and repeatable. Their error is systematic (e.g., misread cylinder volume or improper balance zero). Once that calibration error is corrected, their data will be both accurate and precise. Team Gamma, by contrast, has huge random scatter (range of 1.52 g/cm³); their average matched by pure luck.

Part 3: Error Classification Solutions

Case A: Parallax Reading Meniscus

Error Type: Systematic Error

Primary Impact: Destroys Accuracy (shifts every reading in one direction).

Case B: Air Current / Unshielded Drafts

Error Type: Random Error

Primary Impact: Destroys Precision (causes unpredictable scatter up and down).

Rapid Exit Check Solutions & Scoring Rubric

2 Points Total

Q1: Can measurements be precise without being accurate? Example? (1 pt)

Yes. Example: Weighing a sample 3 times on a balance that was not tared (e.g., getting 15.21 g, 15.22 g, 15.21 g when actual mass is 12.00 g). The measurements cluster tightly (precise) but are all incorrect by the weight of the weigh boat (inaccurate).

Q2: Water boiling at 104.1°C, 104.2°C, 104.0°C, 104.1°C (accepted: 100.0°C): (1 pt)

Correct Choice: Low Accuracy, High Precision Range = 0.2°C (very tight); Mean = 104.1°C (+4.1°C error)

Target Practice: Accuracy vs. Precision — Teacher Solutions Key Page 2 of 2

Unpredictable variations that scatter data above and below true value.

• Air drafts moving an analytical balance pan.
• Fluctuating room temperatures.
• Impact: Destroys Precision. (Cannot repeat data closely).

Pro Tip: Repeating trials 10 times reduces random error, but does NOT fix systematic bias!

Calculating Laboratory Error

Practice Pacing: 12:00–22:00

Quantifying Accuracy

Percent Error

\[ \text{Percent Error} = \frac{|\text{Exp} - \text{Accepted}|}{\text{Accepted}} \times 100\% \]

The absolute difference between your lab average and the literature value. Standard lab target is < 5%.

Quantifying Precision

Range & Spread

\[ \text{Range} = \text{Value}_{\text{highest}} - \text{Value}_{\text{lowest}} \]

The numerical difference between your extreme trials. A smaller range indicates tighter experimental precision.

Turn to Part 2 of your worksheet to compute Team Beta's percent error. Accepted Density: 8.96 g/cm³

Rapid Exit Check

28:00 - 30:00 Final Check

Question 1

Can a series of measurements be highly precise while possessing zero accuracy? Name one chemistry lab example.

Question 2

Four trials for water's boiling point yield: 104.1°C, 104.2°C, 104.0°C, 104.1°C (True: 100.0°C).

How would you classify this experimental dataset? Be prepared to justify your choice.

Complete the bottom section of your worksheet and hand it in. Target Practice Complete!