Home / AS & A Level Chemistry 2.2 The mole and the Avogadro constant Exam Style Practice Questions Paper 1
What contains \(9.03\times10^{23}\) oxygen atoms?
Correct Answer: \( \boxed{\mathrm{B}} \)
\(9.03\times10^{23}\) oxygen atoms correspond to
\( \frac{9.03\times10^{23}}{6.02\times10^{23}}=1.5\,\mathrm{mol} \)
Compare the number of oxygen atoms supplied by each substance:
Therefore, the correct answer is (B).
An experiment is carried out to determine the value of \(x\) in hydrated lithium hydroxide, \( \mathrm{LiOH\cdot xH_2O} \). A sample of the solid is heated in a crucible over a Bunsen flame.
Complete dehydration takes place; decomposition does not occur.
Mass of empty crucible \(=Q\)
Mass of crucible with \( \mathrm{LiOH\cdot xH_2O} \) \(=R\)
Mass of crucible and residue after heating \(=S\)
Which equation gives the correct value of \(x\)?
Correct Answer: \( \boxed{\mathrm{C}} \)
Mass of hydrated lithium hydroxide:
\( \mathrm{R-Q} \)
Mass of anhydrous \( \mathrm{LiOH} \) remaining after heating:
\( \mathrm{S-Q} \)
Mass of water lost:
\( \mathrm{R-S} \)
Moles of water removed:
\( \mathrm{\dfrac{R-S}{18}} \)
Relative formula mass of \( \mathrm{LiOH} \):
\( \mathrm{6.9+16+1=23.9} \)
Moles of \( \mathrm{LiOH} \):
\( \mathrm{\dfrac{S-Q}{23.9}} \)
Therefore:
\( \mathrm{x=\dfrac{(R-S)/18}{(S-Q)/23.9}=\dfrac{23.9(R-S)}{18(S-Q)}} \)
Hence, the correct answer is (C).
Which statement about \(_{53}^{131} I\) is correct?
Ans: D
For \(_{53}^{131} I\), the number of neutrons = Mass number (131) – Atomic number (53) = 78. A negative ion means the atom gains 1 electron, so the number of electrons = Atomic number (53) + 1 = 54. Thus, the correct statement is D, which states 78 neutrons and 54 electrons.