Weighing Objects Tutorial

Objectives

This tutorial aims to familiarize you with the following principles of weighing:

Choosing the Proper Instrument for Weighing

All weighing in the chemistry laboratory is done with a balance. However, depending on the requirements of the experiment at hand, different balances may be used.

Preparative work, such as measuring out a rough amount of an excess reagent, does not require a high level of accuracy. For example, if an experimental procedure calls for 1.0 grams of a reagent known to be in excess in the reaction, amounts of 0.9 or 1.1 grams are perfectly acceptable. For these purposes, a top-loading balance is used to obtain an initial, approximate mass that is within the range specified by the procedure. The sample's mass is then determined more accurately before being presented in a laboratory report. A top-loading balance is a relatively simple electronic device that measures the mass of an object to an uncertainty of ±0.05 grams, thus being suitable for preparative applications in the chemistry laboratory.

In analytical work, such as precisely determining the mass of a lead salt precipitate meticulously isolated from an aqueous solution, an analytical balance should be used to minimize uncertainty. An analytical balance is a highly sensitive instrument that is much more prone to errors from environmental conditions and requires much more careful operation than a top-loading balance. This balance measures an object's mass to an uncertainty of ±0.00005 grams and is used any time a very accurate mass determination is needed.

Question 1

Determine whether each of the tasks below is best accomplished with a top-loading or an analytical balance.

  1. Measuring the mass of a lead salt precipitate to the nearest 0.1 gram.
  2. Weighing out anhydrous solid copper sulfate for a reaction where the procedure calls for 1.5 grams of this chemical.
  3. Determining the water content of hydrated copper sulfate by weighing a sample of this salt prior to and following drying in an oven.
  4. Weighing out sodium oxalate for a solution that will be used to standardize a solution of potassium permanganate via a Fowler and Bright modified titration.

Technique Notes for More Accurate Weighing

Presented below are some general pointers to increase the accuracy of your mass measurements, no matter which balance you use.

Weigh objects at room temperature. A warm or hot object will create a convection current in the air around the balance pan. This fluctuating force reduces the air pressure on the balance pan and can make it difficult to obtain a stable reading.
Weigh only dry objects. Moisture can corrode the balance pan. Moisture evaporating from your sample can lead to unstable mass readings.
Use approximate amounts and then measure them accurately.

Approximate amounts should be weighed out on a top-loading balance and then weighed accurately on an analytical balance.

For example, if the lab manual asks you to weigh out 0.8 grams of a material, a range of 0.7-0.9 grams as measured by an analytical balance is acceptable for the reaction. It would take much too long to weigh out exactly 0.8000 grams of the material. Instead, weigh your approximately measured out sample on an analytical balance to obtain its accurate mass. This mass, accurate to the nearest 0.1 mg, can be stated in your laboratory reports.
Use the same balance. It is critical to always use the same balance especially if calculations require more than one mass measurement, as each device might be calibrated slightly differently.
Use weigh boats. Weigh boats are containers used to prevent reagents from contacting the balance pan. They are made of polypropylene, a plastic that does not adsorb water. They are inexpensive and do not need to be handled with care. If one is torn or too dirty to be wiped clean, simply discard it.

Using the Top-Loading Balance

Analytical Balance

Provided below is an interactive Macromedia Flash walk-through that shows you the steps in properly using the top-loading balance.


Using the Analytical Balance

Analytical Balance

Analytical balances are highly sensitive instruments and provide very accurate mass measurements. However, this accuracy comes at a price: not only are they expensive, but analytical balances are also quite susceptible to environmental conditions that can affect the mass reading. The table below outlines several events that can cause the mass reading to shift by several ten-thousandths of a gram or be unstable. The remedies for each problem are also shown below.

Source of Error Remedy for Error
The floor of the balance room shaking or tilting very slightly as people move around the room. Place the balance on a heavy table, near an outside wall where floor motions will be minimized.
Leaning on or laying a hand onto the balance table. Place the balance on a heavy table and try to minimize the pressure applied to the table.
Air currents lifting the balance pan. Close the sliding doors on the balance.
Wet samples becoming lighter as the moisture they contain evaporates off. Make sure that the sample is dry before it is weighed.
Ultra-dry samples becoming heavier as they adsorb moisture from the atmosphere. Let the sample sit at room temperature for five minutes prior to weighing it.
Weighing warm objects creates a convection current that lifts the balance pan. Let the sample sit at room temperature for five minutes prior to weighing it.
Handling glass objects with bare fingers deposits oil on them and increases their mass. Hold object through a strip of paper or wear latex gloves.
Breathing on a glass object condenses moisture on it and increases its mass. Don't breathe on the object.

The most accurate reading on an analytical balance will be the first stable weight displayed. The most accurate reading is not necessarily the value that seems to persist for a longer time.

The interactive Macromedia Flash walk-through provided below shows you the steps in properly using the analytical balance.


Weighing by Differences

Weigh Boat

Weighing by differences is a technique that is used when it is important to know the precise amount of a sample that has been delivered into a reaction mixture. For this purpose, It is insufficient to simply tare a weigh boat and accurately weigh the sample that has been transferred into it, because some of the sample inevitably sticks to the weigh boat and does not make it into the reaction beaker on a subsequent transfer. Weighing by differences tells you exactly how much of the sample made it out of the weigh boat.

The interactive Macromedia Flash walk-through provided below shows you the steps for weighing a sample by differences.


Question 2

You are weighing out a sample of copper sulfate by differences. You obtain a weigh boat, place it onto a top-loading balance, tare the balance with the empty weigh boat, and measure out your sample. Satisfied that your sample's weight on the top-loading balance is within the range specified by the lab manual, you complete the weighing by differences on an analytical balance and transfer a precisely measured amount of the copper sulfate into a reaction beaker.

Your lab partner comes up to you and informs you that you accidentally did your weighing with her used weigh boat, which contained small amounts of copper sulfate. How does this affect your result? Do you need to repeat your measurement?

Summary

This tutorial on weighing objects has presented the following topics:

  1. How to choose between a top-loading and an analytical balance.
  2. Technical pointers for more accurate weighing.
  3. Correct technique for weighing with a top-loading balance.
  4. Correct technique for weighing with an analytical balance.
  5. The advantages of and correct technique for weighing by differences.