The two most important principles that must be fulfilled whenever you work in a chemical laboratory are:
1) BE ACCURATE and
2) STRICTLY OBEY SAFETY RULES.
For this reason always bear these principles in mind:
1. Keep your working place in a good order and don’t change it without the permission from the instructor.
2. Before starting the experiment, read the manual and listen to the instructor, be quite sure that all the theoretical and experimental aspects are clear to you.
3. Check that all the equipment and chemicals required are on your bench. If no, approach the instructor or the technician. Never take any item from other benches.
4. Make sure that all the equipment you are going to work with is clean and if necessary, clean it.
5. All the experimental observations and quantitative data should be recorded immediately.
6. Strictly follow the procedure.
7. After finishing the experiment, clean all the equipment you used and your working place.
1. Handling the Bunsen burner
a) Don’t work with a burner if the rubber tubing connecting it to the gas tap is damaged or loose;
b) don’t keep the gas tap open while waiting to the lighter;
c) close the gate valve at the end of the laboratory session;
d) if you smell the gas while the tap is closed, inform the instructor immediately.
2. Handling chemical reagents
a) Be careful with corrosive substances (like acids, bases, organic solvents), and particularly careful with their concentrated solutions;
b) if a corrosive substance is dropped on your skin, wash it immediately with water and inform the instructor without delay;
c) if you dilute concentrated sulfuric acid always pour acid to water; newer do the opposite;
d) use spatulas for taking solid reagents - not hands;
e) never close a test tube with a finger - use a cork;
f) don’t taste anything in the laboratory;
g) if you need to smell a substance don’t put your nose into the container; keep the container at some distance and by waving motions of your palm above the mouth of the container send the vapors of the substance towards your nose;
h) reactions involving dangerous or unpleasant odors are to be performed in a hood.
3. Handling glassware
a) Hold the glassware firmly but not squeezing it;
b) do not use broken or cracked glassware;
c) if the glassware is accidently broken, the remainings must be swept to the waste bin immediately.
4. Heating procedure
a) The mouth of a test tube mustn’t point at yourself or anybody else;
b) don’t stand too close to the apparatus in which material is being heated, but never leave it;
c) don’t hold a test tube in your hands while heating, use test tube holders.
CHEMICAL EQUIVALENT. LAW OF EQUIVALENTS
According to the law of equivalents, all the chemicals react with each other in the amounts which are proportional to their chemical equivalents.
Chemical equivalent is the amount of a substance (in moles) which can react with 1 mole of hydrogen atoms or replace its same amount from a chemical compound.
Equivalent mass, ÌE is the mass of one equivalent of a substance expressed in grams per mole.
Calculation of equivalent masses if compounds of different classes can be performed using the following formulae:
(i) elements in free state and in chemical compounds:
ÌE = Ì / V (V is valency of the element)
(ii) acids and bases: ÌE = Ì / n (n is basidity of the acid or acidity of the base, means number of Í+ or ÎÍ- ions)
(iii) oxides and salts: ÌE = Ì / p q ( p is number od metallic atoms and q is their valency)
Number of equivalents nE = m / ME ( m is mass of the sample).
For gaseous substances
nE = V0 / VE0 (V0 is molar volume of the gas under normal conditions, V0 = 22.4 l × mole-1)
VE0 is equivalent volume of the gas, means the volume which is occupied by one equivalent of a gas under normal conditions.
Possible ways of mathematical expression of law of equivalents:
nE1= nE2 ; M1 / ME 1 = Ì2 / ME 2 ; V01 / VE01 = V02 / VE02
QUESTIONS AND PROBLEMS
1. A metal hydride contains 2.02 g of hydrogen and 13.88 g of metal. Calculate equivalent mass of the metal.
2. While 53.96 g of a metal is oxidized 101.96 g of an oxide is formed. Calculate equivalent mass of the metal.
3. 4.80 g of Ca and 7.85 g of Zn replace the same amount of hydrogen from an acid. Calculate equivalent mass of zinc if the equivalent mass of calcium equals 20.0 g×mole-1.
4. Calculate equivalent masses of a metal and sulfur if 4.86 g of the metal form 5.22 g of oxide and 5.58 g of sulfide.
5. While 0.595 g of an unknown substance reacts with 0.275 g of hydrogen chloride, 0.440 g of a salt is formed. Calculate equivalent masses of the substance and the salt.
6. The volume of 2800 ml of hydrogen measured under normal conditions can reduce 11.75 g of a metal oxide. Calculate equivalent masses of the metal and its oxide.