Which one among the following oxides has the highest melting point ?
- A.Na2O
- B.MgO
- C.Fe2O3
- D.CuO
Correct answer
Explanation
The correct answer is B, MgO. Magnesium oxide melts at about 2,850 °C, far higher than the other three, because its small Mg2+ and O2− ions carry double charges and hold each other with a very strong ionic lattice. The strength of an ionic lattice rises with the charge on the ions and falls with their size. Magnesium oxide has both high charges and small ions, which is why it is used as a refractory lining in furnaces and as an insulator in heating elements. Option A is wrong because sodium oxide is built from singly charged Na+ ions, so its lattice is weaker and it melts at only about 1,130 °C. Option C is wrong because iron(III) oxide melts at about 1,560 °C and decomposes near that temperature. Option D is wrong because copper(II) oxide melts at about 1,330 °C, also well below magnesium oxide. Exam tip: high ionic charge + small ions = high melting point; MgO is a classic refractory.
Practice Questions
View allEqual volumes of all gases, under the same conditions of temperature and pressure, contain an equal number of molecules. This statement is known as
- A.Avogadro's law
- B.Boyle's law
- C.Charles's law
- D.Dalton's law of partial pressures
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It is the reason one mole of every gas occupies the same 22.4 litres at standard temperature and pressure, and it allowed chemists to settle the formulae of gases such as hydrogen and oxygen as having two atoms each.
Read the full explanation →Carbon forms two oxides, carbon monoxide and carbon dioxide, in which twelve grams of carbon combines with sixteen grams and thirty two grams of oxygen respectively. This illustrates
- A.The law of conservation of mass
- B.The law of constant proportions
- C.The law of multiple proportions
- D.Avogadro's law
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When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other bear a simple whole number ratio, and here the oxygen masses of sixteen and thirty two grams for the same twelve grams of carbon are in the ratio…
Read the full explanation →How many moles are present in 36 grams of water?
- A.One mole
- B.Two moles
- C.Three moles
- D.Half a mole
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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.
Read the full explanation →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
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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…
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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