What personal protective equipment should you wear when preparing a laboratory?

Wear a lab coat, chemical splash goggles, and gloves to protect yourself from chemical hazards during solution preparation. These items shield your skin, eyes, and clothing from potential splashes or spills when handling reagents like erythrosin B dye and other chemicals throughout the experiment.

How do you prepare a 1.5 × 10⁻⁵ M erythrosin B solution?

Weigh 0.0066 g of erythrosin B on an analytical balance and transfer it to a 250-mL Erlenmeyer flask. Add 200 mL of deionized water, stir until homogeneous, then transfer to a 500-mL volumetric flask and fill to the line with deionized water. Invert several times to ensure thorough mixing and proper concentration.

Why is a volumetric flask used when preparing solutions?

A volumetric flask ensures precise solution concentration by allowing you to fill to an exact volume mark with accuracy. This precision is critical for reproducible experimental results, especially when preparing dilute solutions like erythrosin B dye at specific molarity concentrations for consistent lab outcomes.

What glassware and equipment should be set up at each student lab station?

Each station requires beakers (50, 100, 250, 400, 600 mL), a 500-mL filter flask, Büchner funnel, graduated cylinders, volumetric pipettes, volumetric flask, watch glass, stirring rod, rubber policeman, and filter paper. Additional items include tubing, ruler, rubber adapter, stopper, string, and disposable pipette for independent student work.

How should you store the prepared erythrosin B dye solution?

Transfer the solution into a labeled 500-mL squeeze bottle indicating its concentration and store it on a mat in the hood. This storage method protects the solution from light exposure and contamination while keeping it accessible for student use during the experiment.

What materials should be prepared for student access during the lab?

Place acetone in an accessible area, with approximately 40 mL per student. Set out weigh boats, scoops, and sand (about 3 g per student) next to a balance. Position an organic waste container in a central hood for safe disposal of chemical materials used throughout the experiment.

Scientific Measurement and Lab Skills

Concepts Instructor Prep Student Protocol

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Translating experimental findings from data points to visual representations, like graphs, is essential to determining the relationship between two or more properties. These properties are called variables. When there are two variables, a graph created from the data is called two-dimensional. The graph has two axes. The independent variable is plotted on the x-axis, and the dependent variable is plotted on the y-axis.

Take, for example, this sample data for the temperature and volume of a gas. The volume of the gas depends on the temperature. Thus, we would plot the measured temperature on the x-axis and the volume on the y-axis.

When there are several data points with the same x-value, like if we measured the volume several times at one temperature, we also calculate the standard deviation of those measurements. The standard deviation is a statistical value, which indicates the amount of variation present in a set of values.

Standard deviation is calculated using this formula, where n is the number of data points, x bar is the mean value of the data points, and x_i represents each individual data point. You can compute the standard deviation by hand, or a spreadsheet program can calculate it automatically. The closer the standard deviation is to 0, the closer the data points are to the mean value. If the standard deviation is equal to 0, then all of the entered data points have the same value.

Let's look at the mean values and standard deviations of the volume measurements at each temperature in our data set. We can summarize each set of data points as the mean plus or minus the standard deviation. We determine the significant figures for each mean from the corresponding measurements and round the mean values accordingly.

The standard deviation of each group must have the same number of decimal places as the mean, so we round each standard deviation to the hundredths place. To graphically determine the relationship between two variables, we can fit the data with a best-fit function.

The function is automatically generated by spreadsheet software and can take the form of a linear trend line, a polynomial function, or an exponential or logarithmic function. In the case of our temperature and volume data, the relationship is a linear one. So, the data points are fit by linear least squares regression. Your spreadsheet program will return the equation for the best fit line and an r-squared value. The closer the r-squared value is to 1, the better the fit of the data.

Next, you can use your spreadsheet software to find the standard deviations of the slope, the y-intercept, and the calculated y-value. To determine the significant figures for the values in the equation, we follow a simple rule. The last significant figure of each value corresponds to the first significant decimal place of its standard deviation.

Thus, we round the slope to the thousandths place and the y-intercept to the tenths place, and we round the standard deviations to match. Our slope is 0.167 +/- 0.003 liters per Kelvin, and our y-intercept is -40.6 +/- 1.2 liters. Any calculated y-value will be rounded to the tenths place and will be +/- 0.8 liters. This equation describes the relationship between temperature and the volume of a gas.

In this lab, you will create a data set of dependent and independent variables by measuring the diameter and circumference of various sizes of beakers. You'll then use this data to create a scatter plot and perform a linear regression, keeping in mind the importance of significant figures. You'll also practice lab skills, like filtering and measuring volume using pipettes, paying attention to the uncertainty in the measurements and analysis.

At the end of this lab, students should know...

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How can you visualize the relationship between two measured variables?

To display the relationship between two variables, plot the data points with the dependent variable on the y-axis and the independent variable on the x-axis. This is called a two-dimensional graph.