The mass of 0.5 mole of N₂ gas is
- A.7 g
- B.14 g
- C.21 g
- D.28 g
Correct answer
Explanation
The correct answer is B, 14 g. One mole of N₂ weighs 28 g, so half a mole weighs 14 g.
A nitrogen molecule is made of two nitrogen atoms, each with an atomic mass of 14 u, so its molecular mass is 28 u and its molar mass is 28 g per mole. Mass equals the number of moles multiplied by the molar mass: 0.5 × 28 = 14 g. One mole of any substance contains the Avogadro number of particles, about 6.022 × 10²³, so 14 g of nitrogen gas holds about 3.011 × 10²³ molecules. At standard temperature and pressure one mole of a gas fills about 22.4 litres, so this amount would fill about 11.2 litres.
A is wrong: 7 g is half the mass of one mole of nitrogen atoms, not molecules; it is 0.25 mole of N₂.
C is wrong: 21 g would be 0.75 mole of N₂.
D is wrong: 28 g is the mass of a full mole of N₂.
Exam tip: mass = moles × molar mass; for diatomic gases like N₂ and O₂, double the atomic mass first.
Practice Questions
View allWhich one of the following is the 'energy currency' for cellular processes?
- A.Glucose
- B.ATP
- C.ADP
- D.Pyruvic acid
Show answer
Adenosine triphosphate stores energy in its phosphate bonds and releases it wherever the cell needs it, so it is called the energy currency of the cell.
Read the full explanation →Which one of the following is the correct reactivity order of metals reacting with dilute HCl?
- A.Mg > Al > Zn > Fe
- B.Mg < Al < Zn < Fe
- C.Mg > Zn > Fe > Al
- D.Fe > Mg > Al > Zn
Show answer
This is the order in which these metals stand in the reactivity series, and it matches how vigorously they release hydrogen from dilute hydrochloric acid.
Read the full explanation →Deficiency of iron causes
- A.Anaemia
- B.Scurvy
- C.Rickets
- D.Cholera
Show answer
Iron is needed to make haemoglobin, so too little iron means too little haemoglobin and a reduced ability of the blood to carry oxygen.
Read the full explanation →Consider the following statements regarding waves : 1. If the particles of a medium move along the direction of motion of the wave, the wave is called a longitudinal wave. 2. If the particles of a medium move perpendicular to the direction of motion of the wave, the wave is called a transverse wave. 3. Transverse waves may involve change in the shape of the medium, whereas longitudinal waves involve change in the density of the medium. Which of the statements given above is/are correct ?
- A.1 and 2 only
- B.1 only
- C.2 and 3 only
- D.1, 2 and 3
Show answer
All three statements correctly describe how particles move in the two kinds of mechanical waves. In a longitudinal wave, such as sound in air, the particles vibrate back and forth along the direction in which the wave travels.
Read the full explanation →In paper manufacturing, degumming of the raw material is done using
- A.sulphuric acid
- B.bleaching powder
- C.caustic soda
- D.nitric acid
Show answer
In paper making the raw plant material is cooked with caustic soda, which dissolves the gums, resins and lignin that bind the cellulose fibres together.
Read the full explanation →