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How many moles are present in 36 grams of water?

  1. A.One mole
  2. B.Two moles
  3. C.Three moles
  4. D.Half a mole

Correct answer

B. Two moles

Explanation

The correct answer is B, two moles. The molecular mass of water is eighteen unified atomic mass units, two hydrogen atoms of 1 u each and one oxygen atom of 16 u, so one mole of water weighs eighteen grams. The number of moles is the given mass divided by the molar mass, that is thirty six divided by eighteen, which gives two. Those two moles contain twice the Avogadro constant of water molecules, that is about 1.2 into ten to the power twenty-four molecules. Option A is wrong because one mole would weigh only eighteen grams. Option C is wrong because three moles would weigh fifty four grams. Option D is wrong because half a mole weighs nine grams. The same single formula, moles equal mass divided by molar mass, answers every question of this family, so practise it with carbon dioxide at forty four grams and sodium chloride at 58.5 grams for one mole each.

Read the full article: Atoms, Molecules and the Mole Concept: Exam Notes

Practice Questions

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Q1.General ScienceEasy

The law of conservation of mass in a chemical reaction was given by

  1. A.John Dalton
  2. B.Antoine Lavoisier
  3. C.Joseph Proust
  4. D.Amedeo Avogadro
Show answer
Correct answer: B. Antoine Lavoisier

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.

Q2.General ScienceMedium

The law which states that a pure chemical compound always contains the same elements combined in the same proportion by mass is known as

  1. A.The law of multiple proportions
  2. B.The law of constant proportions
  3. C.The law of conservation of mass
  4. D.The law of gaseous volumes
Show answer
Correct answer: B. The law of constant proportions

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.

Q3.General ScienceAsked in: RRB ALP · 17 Aug 2018, Shift 2Easy

Atomic mass of Nitrogen is 14 and that of Hydrogen is 1. What will be the molecular mass of Ammonia?

  1. A.17
  2. B.18
  3. C.15
  4. D.16
Show answer
Correct answer: A. 17

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.

Q4.General ScienceAsked in: RRB ALP · 27 Nov 2024, Shift 2Medium

The formula unit mass of CaCl2 is:

  1. A.115u
  2. B.114u
  3. C.111u
  4. D.110u
Show answer
Correct answer: 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.

Q5.General ScienceEasy

The number of particles present in one mole of any substance is

  1. A.6.022 into ten to the power twenty-three
  2. B.6.022 into ten to the power twenty-one
  3. C.3.011 into ten to the power twenty-three
  4. D.9.8 into ten to the power twenty-three
Show answer
Correct answer: A. 6.022 into ten to the power twenty-three

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.