The formula unit mass of CaCl2 is:
- A.115u
- B.114u
- C.111u
- D.110u
Correct answer
Explanation
The correct answer is C, 111u. Calcium chloride is an ionic compound, so the sum of the atomic masses in its formula is called the formula unit mass rather than the molecular mass. Calcium has an atomic mass of 40 u and chlorine of 35.5 u, so the sum is forty plus two times thirty five and a half, which is one hundred and eleven unified atomic mass units. Option A is wrong because 115 u would need chlorine to be taken as 37.5 u. Option B is wrong because 114 u comes from using 37 u for chlorine, the mass of its heavier isotope rather than the average. Option D is wrong because 110 u is the figure you get if chlorine is rounded down to 35 u, which is the single commonest mistake in this sum. Chlorine is given as 35.5 u because it occurs as two isotopes of mass 35 and 37 in roughly a three to one ratio, and the average is what appears in the periodic table.
Read the full article: Atoms, Molecules and the Mole Concept: Exam Notes
Practice Questions
View allThe law of conservation of mass in a chemical reaction was given by
- A.John Dalton
- B.Antoine Lavoisier
- C.Joseph Proust
- D.Amedeo Avogadro
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The correct answer is B, Antoine Lavoisier, the French chemist who showed by careful weighing in closed vessels that the total mass of the products of a reaction is exactly equal to the total mass of the reactants, so that matter is neither created nor destroyed in a chemical change.
Read the full explanation →The law which states that a pure chemical compound always contains the same elements combined in the same proportion by mass is known as
- A.The law of multiple proportions
- B.The law of constant proportions
- C.The law of conservation of mass
- D.The law of gaseous volumes
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Water drawn from a river, from rain or made in a laboratory always has hydrogen and oxygen in the mass ratio of one to eight, and ammonia always has nitrogen and hydrogen in the ratio of fourteen to three.
Read the full explanation →Atomic mass of Nitrogen is 14 and that of Hydrogen is 1. What will be the molecular mass of Ammonia?
- A.17
- B.18
- C.15
- D.16
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The formula of ammonia is NH3, so its molecular mass is the atomic mass of one nitrogen atom added to the atomic masses of three hydrogen atoms, that is fourteen plus three times one, which equals seventeen unified atomic mass units; one mole of ammonia therefore weighs seventeen grams.
Read the full explanation →The number of particles present in one mole of any substance is
- A.6.022 into ten to the power twenty-three
- B.6.022 into ten to the power twenty-one
- C.3.011 into ten to the power twenty-three
- D.9.8 into ten to the power twenty-three
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It is the number of atoms in twelve grams of carbon twelve, and it holds for atoms, molecules, ions or electrons alike, so one mole of water has that many water molecules and one mole of sodium has that many sodium atoms.
Read the full explanation →At standard temperature and pressure, the volume occupied by one mole of any gas is
- A.11.2 litres
- B.22.4 litres
- C.24.4 litres
- D.1 litre
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It follows from Avogadro's law: since equal volumes of all gases at the same temperature and pressure contain an equal number of molecules, one mole of hydrogen, of oxygen or of carbon dioxide all take up the same space at standard temperature and pressure, even though their masses are two…
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