Online Test — Some Basic Concepts of Chemistry
20 Questions • 15 min • Chapter MCQ
15:00
Question 1 of 20
The value of the Avogadro constant is:
$6.022\times10^{22}$
$6.022\times10^{23}$
$3.011\times10^{23}$
$1.602\times10^{19}$
Explanation: $N_A = 6.022\times10^{23}$ entities per mole.
Question 2 of 20
The number of moles in $44\,g$ of $CO_2$ is:
$0.5$
$1$
$2$
$4$
Explanation: Molar mass of $CO_2 = 44\,g\,mol^{-1}$; $n = 44/44 = 1\,mol$.
Question 3 of 20
The law of definite proportions was proposed by:
Lavoisier
Dalton
Proust
Avogadro
Explanation: Proust stated that a compound always has the same fixed mass composition.
Question 4 of 20
The molecular formula of a compound with empirical formula $CH_2$ and molar mass $42\,g\,mol^{-1}$ is:
$C_2H_4$
$C_3H_6$
$C_4H_8$
$CH_2$
Explanation: Empirical mass $= 14$; $n = 42/14 = 3$; formula $= C_3H_6$.
Question 5 of 20
Which reactant fixes the maximum amount of product formed in a reaction?
the excess reagent
the catalyst
the limiting reagent
the solvent
Explanation: The limiting reagent is fully consumed and determines the yield.
Question 6 of 20
The molarity of a solution with $0.5\,mol$ solute in $2\,L$ is:
$0.25\,M$
$0.5\,M$
$1\,M$
$2\,M$
Explanation: $M = 0.5/2 = 0.25\,mol\,L^{-1}$.
Question 7 of 20
Equal volumes of gases at the same T and P contain equal numbers of molecules. This is:
Dalton's law
Avogadro's law
Proust's law
Boyle's law
Explanation: This is Avogadro's law.
Question 8 of 20
The mass of $2\,mol$ of $NaOH$ is:
$20\,g$
$40\,g$
$80\,g$
$160\,g$
Explanation: Molar mass $= 40$; mass $= 2\times40 = 80\,g$.
Question 9 of 20
Which concentration term does NOT change with temperature?
Molarity
Molality
Normality
Formality
Explanation: Molality uses mass of solvent, which is temperature-independent.
Question 10 of 20
The number of molecules in $0.5\,mol$ of $N_2$ is:
$3.011\times10^{23}$
$6.022\times10^{23}$
$1.204\times10^{24}$
$3.011\times10^{22}$
Explanation: $N = 0.5\times6.022\times10^{23} = 3.011\times10^{23}$.
Question 11 of 20
The volume occupied by $1\,mol$ of any ideal gas at STP is:
$11.2\,L$
$22.4\,L$
$24.0\,L$
$44.8\,L$
Explanation: Molar gas volume at STP is $22.4\,L$.
Question 12 of 20
The percentage of nitrogen in urea, $CO(NH_2)_2$ (M = 60), is:
$23.3\%$
$28\%$
$46.7\%$
$60\%$
Explanation: Nitrogen mass $= 28$; $\%\,N = (28/60)\times100 = 46.7\%$.
Question 13 of 20
On diluting $100\,mL$ of $1\,M$ solution to $500\,mL$, the molarity becomes:
$0.2\,M$
$0.5\,M$
$1\,M$
$5\,M$
Explanation: $M_1V_1 = M_2V_2 \Rightarrow 1\times100 = M_2\times500 \Rightarrow M_2 = 0.2\,M$.
Question 14 of 20
When $1\,mol$ $H_2$ reacts with $1\,mol$ $Cl_2$ ($H_2 + Cl_2 \rightarrow 2HCl$), the limiting reagent is:
$H_2$
$Cl_2$
neither (exact ratio)
$HCl$
Explanation: The mole ratio is 1:1, matching the equation exactly, so neither is limiting.
Question 15 of 20
The mole fraction of water in a solution of $1\,mol$ ethanol and $3\,mol$ water is:
$0.25$
$0.5$
$0.75$
$1.0$
Explanation: $x_{water} = 3/(1+3) = 0.75$.
Question 16 of 20
The empirical formula of glucose, $C_6H_{12}O_6$, is:
$CHO$
$CH_2O$
$C_2H_4O_2$
$C_3H_6O_3$
Explanation: Dividing subscripts by 6 gives $CH_2O$.
Question 17 of 20
The total mass of products equals the total mass of reactants. This is the law of:
conservation of mass
multiple proportions
combining volumes
definite proportions
Explanation: This is the law of conservation of mass.
Question 18 of 20
A water sample has $2\,g$ of solute in $1000\,g$ of solution. Its concentration in ppm is:
$200\,ppm$
$500\,ppm$
$1000\,ppm$
$2000\,ppm$
Explanation: $ppm = (2/1000)\times10^{6} = 2000\,ppm$.
Question 19 of 20
The number of significant figures in the measurement $0.00203\,kg$ is:
$2$
$3$
$5$
$6$
Explanation: Leading zeros are not significant; $2$, $0$ and $3$ are — three significant figures.
Question 20 of 20
The molar mass of $H_2SO_4$ is:
$80\,g\,mol^{-1}$
$90\,g\,mol^{-1}$
$98\,g\,mol^{-1}$
$108\,g\,mol^{-1}$
Explanation: $2(1) + 32 + 4(16) = 2 + 32 + 64 = 98\,g\,mol^{-1}$.