1.7 g of silver nitrate is dissolved in 100 g of water, and 0.585 g of sodium chloride is dissolved in 100 g of water. Both are mixed and a chemical reaction occurs. 1.435 g of silver chloride and 0.85 g of sodium nitrate are formed. Which law of chemical combination is shown by the above reaction?
- A.Law of multiple proportions
- B.Law of conservation of mass
- C.Law of reciprocal proportions
- D.Law of constant proportions
Correct answer
Explanation
The correct answer is B, Law of conservation of mass. The total mass of the reactants equals the total mass of the products, which is exactly what this law states.
The reactants weigh 1.7 g of silver nitrate plus 0.585 g of sodium chloride, a total of 2.285 g. The products weigh 1.435 g of silver chloride plus 0.85 g of sodium nitrate, also 2.285 g. The 200 g of water takes no part in the reaction and is the same before and after. Antoine Lavoisier stated the law in the eighteenth century: in a chemical reaction, mass is neither created nor destroyed.
A is wrong because the law of multiple proportions, given by Dalton, deals with two elements forming more than one compound, such as CO and CO2.
C is wrong because the law of reciprocal proportions compares the ratios in which two elements combine with a third element.
D is wrong because the law of constant proportions says one compound always has the same elements in the same ratio by mass; the data here only compare total masses.
Exam tip: conservation of mass is Lavoisier; constant proportions is Proust; multiple proportions is Dalton.
Practice Questions
View allConsider the following statements : 1. The circulation of nitrogen from the inorganic compounds in the soil to form protein in plants and animals and back again is known as the nitrogen cycle. 2. The preparation of ammonia by Haber process is an endothermic reaction. 3. Urea is synthetically made by the action of ammonia and carbon dioxide under high temperature and high pressure. Which of the statements given above is/are not correct?
- A.1 and 3
- B.2 and 3
- C.2 only
- D.1 only
Show answer
The Haber process is exothermic, not endothermic: nitrogen and hydrogen combine to form ammonia and release heat. In the Haber process, N2 + 3H2 ⇌ 2NH3 gives out about 92 kJ for every two moles of ammonia formed.
Read the full explanation →Which one of the following statements about xylem vessels in plants is not correct?
- A.Xylem vessels have lignified cell walls.
- B.Phloem sieve tubes are identical to xylem vessels in structure.
- C.Dead xylem vessels provide structural support to plants.
- D.Water is transported through xylem vessels.
Show answer
Phloem sieve tubes are not like xylem vessels: sieve tubes are living cells with thin walls and sieve plates, while mature xylem vessels are dead, hollow tubes with thick lignified walls.
Read the full explanation →Which one of the following effects best explains the high partial pressure of carbon dioxide causing haemoglobin to release oxygen in actively respiring tissues?
- A.Haldane effect
- B.Allee effect
- C.Bohr effect
- D.Eagle effect
Show answer
The Bohr effect is the fall in haemoglobin's affinity for oxygen when carbon dioxide and acidity rise, so oxygen is unloaded where tissues need it most.
Read the full explanation →Which one of the following statements about arteries is not correct?
- A.The inner lining is made of squamous endothelium.
- B.The middle layer is made of smooth muscle and elastic fibres.
- C.The outer layer is made of fibrous connective tissue with collagen fibres.
- D.Arteries have the thinnest walls amongst any other blood vessels.
Show answer
Arteries have the thickest walls of all blood vessels, not the thinnest, because they carry blood away from the heart under high pressure.
Read the full explanation →Which one of the following is not an exothermic reaction?
- A.Dissolution of calcium oxide in water
- B.Decomposition of vegetable matter into compost
- C.Decomposition of lead nitrate
- D.Dilution of sodium hydroxide with water
Show answer
Lead nitrate breaks down only when it is heated strongly, so the reaction absorbs heat and is endothermic. On heating, lead nitrate gives lead oxide, brown fumes of nitrogen dioxide and oxygen: 2Pb(NO3)2 → 2PbO + 4NO2 + O2.
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