One unified atomic mass unit, written as u, is defined as
- A.The mass of one hydrogen atom
- B.One twelfth of the mass of one atom of carbon twelve
- C.One sixteenth of the mass of one atom of oxygen
- D.The mass of one electron
Correct answer
Explanation
The correct answer is B, one twelfth of the mass of one atom of carbon twelve. This standard was adopted so that the atomic masses of all elements could be compared on one relative scale, and on it hydrogen comes out as about 1 u, oxygen as 16 u and chlorine as 35.5 u. Option A is wrong because the hydrogen atom was the standard in the earliest scale, but it was given up because masses measured against it did not fit the later data well. Option C is wrong because one sixteenth of an oxygen atom was the standard used before 1961, and chemists and physicists disagreed over which oxygen to use, which is exactly why carbon twelve replaced it. Option D is wrong because the electron is about eighteen hundred times lighter than a hydrogen atom and makes a useless unit for atomic masses. The history of the three standards, hydrogen, then oxygen, then carbon twelve, is itself asked in objective papers.
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 formula unit mass of CaCl2 is:
- A.115u
- B.114u
- C.111u
- D.110u
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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.
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.
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