Measurement

Activity 3: Pre-Assessment

1. What are the conversions for the following measurements?
   1. 50 ml = ____ L
   2. 20 g = ____ mg
   3. 700 μl = ____ ml
   4. 1 gal = ____ pt
   5. 2000 ml = ____ L
2. Visit the weighing station. What is the weight of a weighing boat? Some balances have a method of subtracting it by pressing the “zero” or “tare” button. Weigh three items. Record the results in your notebook.
3. Discuss the answers to questions 1 and 2 with the class.

Activity 3: Measurement

In the lab, you will need to use instruments to make your measurements. There are instruments to use for each type of substance (liquid or solid). Some of the instruments to measure volume are graduated cylinders (shown in Figure 1.2) and pipettes (shown in Figure 1.3 and Figure 1.4). Some instruments to measure solids are balances and scales (a scale is shown in Figure 1.5). You may also need to adjust the instrument or container you are using based on the amount of substance you need to measure.

Also, when recording your measurements in a data table, it is important to be aware of significant figures. The number of significant figures is the number of digits needed to express the accuracy of a calculation. The rules for significant figures are presented in Figure 1.5. To use these rules, let’s look at an example. An experiment requires you to measure the weight of 10 marbles. The scale shows a weight of 32.46547g. All the digits in this measurement are significant figures. However, in the number 32.465470, how do we know if the last 0 is significant or not? There are three rules on how to determine significant figures:

• Non-zero digits are always significant.
• Any zeros between two significant digits are significant.
• A final zero or trailing zeros in the decimal portion only are significant.

So, based on this rule, the final zero in 32.465470 is significant, as is the first zero in 32.046570. However, the zero in 0.324657 would not be significant because it is simply a placeholder.

Safety Precautions

• Inform your teacher immediately of any broken glassware, as it could cause injuries.
• Clean up any spilled fluids to prevent other people from slipping.
• Wash your hands with soap and water after completion of the activity.

For this activity, you will need the following:

• A balance
• A weighing boat
• A graduated cylinder
• Weighable objects

For this activity, you will work in pairs.

Structured Inquiry

Step 1: Create a data table for your measurements and show your calculations in your notebook.

Step 2: Hypothesize/Predict: Examine one of the objects. Predict the object’s mass and write the predicted mass in your notebook.

Step 3: Student-Led Planning: Listen to your teacher’s instructions on how to use the balance. Discuss with your partner how you should determine the weight of the object and how many significant figures your measurement should have. Write your ideas in your notebook.

Step 4: Determine the weight of the object using the scale. Repeat the measurement twice more and record all results in your notebook.

Step 5: Critical Analysis: How much did your measurements vary among the three trials? Were you able to accurately measure to the amount of significant figures stated in Step 3? Why or why not? Could you have improved your methods to get more accurate measurements or obtain a greater number of significant figures? Why or why not? Discuss with your partner, and then write your answers in your notebook.

Guided Inquiry

Step 1: Hypothesize/Predict: Volume is a measure of how much space matter occupies, and it is used to calculate density (d) where m is the mass of the object and v is the object’s volume.

Pick another object available in your lab and predict whether it has a higher or lower density than the object you used during the Structured Inquiry. Write your prediction in your notebook.

Step 2: Student-Led Planning: How can you calculate the density of your two objects? Devise a plan for finding the density of both objects and present it to your teacher for approval. Create the data tables you need and decide how many times you should measure each object as well as how many significant figures you will use.

Step 3: After your teacher approves, take the measurements you need to calculate the density of both objects. Write your final densities in your notebook.

Step 4: Critical Analysis: How did the object’s size influence its mass and its volume? What would you have to do to increase the mass, volume, and density of an object? How could you increase the accuracy of your measurements? Discuss with your partner, and then write your answers in your notebook.

Assessments

1. Why does the weight of the weighing boat need to be subtracted when measuring the weight of an object sitting in the weighing boat?
2. What is the most likely instrument used to measure the following volumes?
   1. 200 ml
   2. 50 μl
   3. 1 L
3. Calculate the density of an object that is 3 g and that displaces water in a graduated cylinder by 3 ml.