Chem106-Exp.1_Mass determin]7ation

Experiment 1: Determination of Mass

Objectives

• - Use the lab balance correctly.

• Determine the mass of some objects using the weigh by difference method.

Background

1. Importance of Mass Measurement

   • Accurate mass determination is essential in experimental chemistry.

   • Mass measures the amount of matter and indicates the number of atoms/molecules.

   • Chemical reactions depend on the mass of reactants used.

2. Types of Balances

   • Different balances with varying precision:

     • Top-loading Balance: Suitable for general weighing.

     • Analytical Balance: Offers higher precision but requires careful handling.

General Usage Guidelines for Balances ‘

1. Zero the Balance

   • Always ensure the balance reads zero before weighing; tare if necessary.

2. Avoid Moisture

   • Moisture can damage balances; clean spills immediately.

3. Proper Weighing Techniques

   • Do not weigh reagents directly on the balance pan.

   • Use a beaker, flask, or weighing boat to contain the reagents.

4. Understanding Weight Requirements

   • When instructed to weigh a specific amount, record the actual mass used within a certain range.

   • Example: Weighing "0.5 g" means a range between 0.45 g and 0.55 g should be used with the actual weight recorded.

5. Object Temperature

   • Objects should be at room temperature to avoid weighing inaccuracies due to air buoyancy.

6. Weighing by Difference

   • This method involves weighing the empty container first, adding the substance, and then determining the mass by finding the difference.

   • This method is more accurate than absolute mass readings.

7. Recording Measurements

   • Always document the balance reading, including its unit and uncertainty.

   • Example: 1.50 g ± 0.01 g.

Apparatus/Reagents Needed

• Unknown mass objects (provided by the instructor).

• Small beakers (2 per student).

• Top-loading balances.

Procedure

1. Preparation

   • Record the codes of objects to be weighed.

2. Initial Weighing

   • Using balance, determine and record the mass of a small beaker (beaker I).

3. Object Weighing

   • Insert the first object into beaker I, measure and record its combined mass.

   • Subtract the mass of the empty beaker to find the mass of the object.

4. Subsequent Measurements

   • Repeat mass determination for all remaining objects in beaker I.

   • Use a different beaker (beaker II) for additional measurements with the same balance.

5. Comparison

   • Compare mass results from both beakers.

Results and Observations

• Record necessary masses for analysis:

  • Mass of empty beaker (beaker I):

  • ID number of object:

  • Mass of (beaker I + object):

  • Mass of object itself:

  • Mass of empty beaker (beaker II):

  • ID number of object:

  • Mass of (beaker II + object):

  • Mass of object itself:

Post-lab Questions

1. Why should reagents never be weighed directly on the balance pan?

2. Why might switching balances lead to errors in mass determination?

3. What effect does placing a warm object on the balance pan have on measurements?