The balance reads to 0.0001 g, so why not weigh NaOH pellets on it? Because the choice of balance depends on the nature of the substance, and that is what this note sorts out.
1. What a chemical balance is
The balance used in volumetric analysis to measure chemicals accurately from the second to the fourth decimal place of a gram (0.01 to 0.0001 g) is called a chemical balance. It weighs accurately up to 0.0001 g.
2. Two kinds of balance
The chemical balance is of two kinds: the Paul Bunge balance and the Sartorius balance.
The beam markings tell them apart. In the Paul Bunge balance the upper beam is divided into 100 parts, with 0 at the left end and 100 at the right end; the balance is levelled with a 5 mg or 10 mg rider placed at the 0 mark. In the Sartorius balance the 0 is in the middle of the beam, with 50 marks on each of the left and right sides.
The Sartorius balance is no longer used and is not in the present syllabus. Today the chemical balance in use is mainly the Paul Bunge balance or the digital balance.
sort themwhere is the zero?
Which balance?
Pick up a chip, then drop it in a box.
Right. Next one.
Not yet. Ask: is the zero at the end or in the middle?
All in place. Zero is at the end in Paul Bunge and in the middle in Sartorius, and Paul Bunge is the one still in use.
3. Which substance on which balance
The chemical balance is used only to weigh a primary standard substance to the fourth decimal place. It is mainly used in college and university chemistry laboratories and in drug manufacturing.
The balance used for the approximate weight of corrosive chemicals is the rough balance. It cannot give the exact weight of a substance.
A solid secondary substance is weighed on a rough balance. NaOH pellets are corrosive, so they go on the rough balance too. Liquid secondary substances such as concentrated HCl and concentrated H₂SO₄ are measured by volume in a measuring cylinder. They cannot be weighed on a chemical balance.
The balance is chosen by the substance. Try each substance in each place, and see what happens and how many decimals the reading gives.
pick the balancea substance, then a place
Where do you measure it?
The readings are made up for the drawing. The book says the chemical balance reads to 0.0001 g and the rough balance is approximate.
Now sort all the substances at once.
sort themthink primary or secondary
How is each measured?
Pick up a chip, then drop it in the box for how it is measured.
Right. Next one.
Not yet. First ask: is it a primary standard or a secondary? If secondary, is it a solid or a liquid?
All in place. Primary standards go on the chemical balance, solid secondaries on the rough balance, liquid secondaries in the measuring cylinder.
4. Quick recall
Short facts blur together after one reading. Think of the answer first, then turn the card.
cut from the notethink, then tap
Balance quick facts
QuestionAccuracy of a chemical balance?
AnswerUp to 0.0001 g.
QuestionHow many kinds of chemical balance?
AnswerTwo.
QuestionWhat does a rough balance give?
AnswerApproximate weight.
QuestionWhich is no longer used?
AnswerSartorius.
QuestionA solid secondary substance is weighed on?
AnswerA rough balance.
QuestionA liquid secondary substance is measured in?
AnswerA measuring cylinder.
5. Check yourself
The first question is on the definition and accuracy of the balance.
check yourselfpick one answer
Question 1
To what accuracy must a balance weigh in volumetric analysis to be called a chemical balance?
Such coarse weighing is not the chemical balance.
Right. The book's definition: second to fourth decimal place of a gram.
This is rough-balance territory. The right answer is: 0.01-0.0001 g.
This is far finer than the book's range.
In the second, match each substance to where it goes.
check yourselfpick one answer
Question 2
In using balances:
i. A primary standard is weighed on a chemical balance ii. Conc. HCl is weighed on a chemical balance iii. A solid secondary substance is weighed on a rough balance
ii is wrong: conc. HCl is measured by volume.
Right. Both are the book's rules.
ii is wrong. Statement ii reads: "Conc. HCl is weighed on a chemical balance".
ii is wrong, so not all three. Statement ii reads: "Conc. HCl is weighed on a chemical balance".
In the last, recall how the two balances differ.
check yourselfpick one answer
Question 3
How are NaOH pellets and conc. H₂SO₄ measured?
Neither conc. H₂SO₄ nor NaOH goes on the chemical balance.
Solid NaOH is weighed; liquid H₂SO₄ is measured by volume.
Right. This is the book's rule.
NaOH pellets cannot be pipetted. The right answer is: NaOH on a rough balance; conc. H₂SO₄ by volume in a measuring cylinder.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionWhat is a chemical balance?
AnswerThe balance that weighs chemicals accurately from the 2nd to the 4th decimal place of a gram (0.01-0.0001 g) in volumetric analysis
QuestionHow is the Paul Bunge balance beam marked?
AnswerIn 100 parts, 0 at the left end and 100 at the right end
QuestionHow is the Sartorius balance beam marked?
Answer0 in the middle and 50 marks on each side
QuestionWhich chemical balance is no longer used?
AnswerThe Sartorius balance
QuestionWhat is a rough balance used for?
AnswerTo take the approximate weight of corrosive chemicals; it does not give exact weight
QuestionOn which balance are NaOH pellets weighed?
AnswerOn a rough balance, as it is a corrosive solid secondary substance
QuestionHow are concentrated HCl and H₂SO₄ measured?
AnswerBy volume in a measuring cylinder; they cannot be weighed on a chemical balance
QuestionUp to what value can a chemical balance weigh accurately?
AnswerUp to 0.0001 g
Source: Chemistry 1st Paper, Classes 11-12 (NCTB), Chapter 1 “Safe Use of the Laboratory”, section 1.4.