Weighing by Difference. Why Weigh by Difference?. The amounts of solid samples weighed in this course are generally small . I.e., often 500 mg or less. Analytical balances can weigh objects with a precision of about 0.2 mg . For a 500 mg sample, this is 100 x 0.2 / 500 = 0.04 %
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The amounts of solid samples weighed in this course are generally small. I.e., often 500 mg or less.
Analytical balances can weigh objects with a precision of about 0.2mg.
We generally strive for better than 1% in precision and accuracy in exercises
What else could keep us from achieving an accuracy of 1%?
The loss of a 10 mg* sample by any means would constitute a
* For comparison, a drop of water weighs about 50 mg.
Problem: The final container (beaker, flask, etc.) almost always exceeds the capacity of an analytical balance (~100g)!
The top loading balance can accommodate such weight, but it provides a precision of only about 20 mg
How can we avoid intermediate containers when using the analytical balance?
Weighing by difference!
Weighing by difference will generally work in this course because substances that need to be weighed with high precision are almost always distributed in sample vials which can safely be put on the balance.
While this method may require several transfers, it will always work and will never cause an excessively heavy object to be put on the balance pan.
1. Weigh the vial and its contents
2. Transfer a sample estimated to weigh 200 mg or less directly to the beaker. (The ability to estimate such amounts will improve with experience.)
3. Reweigh the vial and its contents
14.7936 - 14.6402 = 0.1534 g = 153.4 mg
14.7936 - 14.5783 = 0.2153 g = 215.3 mg
When several weighings of samples of similar sizes are required, the subsequent samples are much more easily estimated by eye.