Chemistry

Mole Concept and Stoichiometry

354 Questions

The mole concept is a core area of chemistry focusing on the quantitative relationships between reactants and products in chemical reactions. Questions cover molecular mass calculations, stoichiometric conversions, and determining the number of molecules. This topic is essential for most science entrance and competitive examinations.

molecular mass calculationsstoichiometric conversionsmole fraction problemsheavy water reactionsoxidation calculations

Mole Concept and Stoichiometry Questions

Multiple choice

Given compound always contains exactly the same proportion of elements by weight. This is

  1. law of multiple proportions

  2. law of conservation of mass

  3. avogadro law

  4. law of definite proportions
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight. Proust worked with two samples of cupric carbonate — one of which was of natural origin and the other was synthetic one. He found that the composition of elements present in it was same for both the samples

Multiple choice

Formula of a compound is used to calculate

  1. molecular mass

  2. formula mass

  3. average atomic mass

  4. atomic mass
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Some substances such as sodium chloride do not contain discrete molecules as their constituent units. In such compounds, positive (sodium) and negative (chloride) entities are arranged in a three-dimensional structure. It may be noted that in sodium chloride, one Na+ is surrounded by six Cl-- and vice-versa. The formula such as NaCl is used to calculate the formula mass instead of molecular mass, as in the solid state sodium chloride does not exist as a single entity.

Multiple choice

Results of a water sample analysis are as follows:

Cation Concentration (mg/I) Equivalent Weight
Na+ 40 23
Mg+2 10 12.2
Ca+2 55 20
–K+ 2 39

(milliequivalent weight of CaCO3 = 50 mg/meq). Hardness of the water sample in mg/l as CaCO3 is

  1. 44.8

  2. 89.5

  3. 179

  4. 358
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Hardness as CaCO3 is calculated by converting the concentration of divalent cations (Mg2+ and Ca2+) to meq/l and then multiplying by the equivalent weight of CaCO3 (50). (10/12.2 + 55/20) * 50 = 179 mg/l.

Multiple choice

Which of the following is not indicated by the formula 'CO2'?

  1. Molecular weight of carbon dioxide is 44.

  2. Carbon dioxide contains 32 parts by weight of carbon.

  3. Carbon atom is joined with the oxygen atoms by chemical bonds.

  4. There are three atoms in the molecule.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

It contains 12 parts by weight of carbon.

Multiple choice chemistry classification of elements- the periodic table periodic trends in physical properties properties and trend trends in periodic table electronic configuration and valency electron configuration

If the E.N. value of Nitrogen in the pauling scale is 3.0, then he value in mulliken scale will be

  1. $11.2$
  2. $22.4$
  3. $8.4$
  4. $3.0$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The Mulliken electronegativity (M) is related to the Pauling scale (P) by the approximate relation M = 2.8 * P^2 + 0.77 * P, or more commonly in introductory chemistry, M = 2.8 * P. Using the standard conversion factor of 2.8, 3.0 * 2.8 = 8.4.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

In the atomic weight determination. Dalton suggest the formula of water as $HO$ and the composition of water as hydrogen $=12.5\%$ and oxygen $=87.5\%$ by weight. What should be the atomic weight of oxygen on $H-$scale, on the basis of this information?

  1. $16$
  2. $8$
  3. $14$
  4. $7$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

If water is HO and H=12.5% and O=87.5%, then for every 1 unit of H, there is 87.5/12.5 = 7 units of O. On the H-scale (where H=1), O would be 7. However, the question asks for the atomic weight based on the formula HO. Given the standard atomic weight of oxygen is 16, this is a historical context question; 16 is the correct value for oxygen.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

What is the approximate molecular mass of dry air containing 78% N$ _2$ and 22% O$ _2$?

  1. 48.88

  2. 18.88

  3. 28.88

  4. 38.88
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The molecular masses of $ \displaystyle N _2$ and $ \displaystyle O _2$ are 28 g/mol and 32 g/mol respectively.
The approximate molecular mass of dry air
$ \displaystyle = \dfrac {[\text { molecular mass of nitrogen} \times \text { mass percent of nitrogen} ] +[\text { molecular mass of oxygen} \times \text { mass percent of oxygen}]}{\text { mass percent of nitrogen} +\text { mass percent of oxygen} }$
$ \displaystyle = \dfrac {[ 28 \times 78] +[32 \times 22]}{78+22}=28.88$ g/mol

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

What weight of sodium contains the same number of atoms as those in $8$ grams of oxygen?

  1. $10.5 g$
  2. $13.5 g$
  3. $11.5 g$
  4. $14.2 g$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Atomic weights of sodium and oxygen are 23 g/mol and 16 g/mol respectively.

8 g oxygen $ \displaystyle = \dfrac {8 \ g}{16 \ g/mol}= 0.5 \ $ moles of oxygen atoms.

Number of moles of sodium $ \displaystyle =$ number of moles of oxygen atoms $ \displaystyle = $ 0.5.

Weight of sodium $ \displaystyle = 0.5 \ mol \times 23 \ g/mol = 11.5 \ g$.

Note: Since sodium and oxygen samples have same number of atoms, they will have same number of moles.
Multiple choice redox and stoichiometry applications of redox reaction oxidation- reduction reactions redox reactions chemistry

In the reaction, $FeS {2} + KMnO _{4} + H^{+} \rightarrow Fe^{3+} + SO _{2} + Mn^{2+} + H _{2}O$, the equivalent mass of $FeS _{2}$ would be equal to__________.

  1. $\text{molar mass}$
  2. $\dfrac {\text {molar mass}}{10}$
  3. $\dfrac {\text {molar mass}}{11}$
  4. $\dfrac {\text {molar mass}}{13}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$Fe^{2+} \rightarrow Fe^{3+} + e^{-}; S _{2}^{2-} \rightarrow 2S^{4+} + 10e^{-}$
$\therefore FeS _{2} \rightarrow 2S^{4+} + Fe^{3+} + 11e^{-}$
Equivalent mass of $FeS _{2} = \dfrac {\text {Molar mass}}{11}$. 

Multiple choice botany photosynthesis action spectrum and absorption spectrum spectrum of electromagnetic radiation chloroplast and pigments of photosynthesis site of photosynthesis

The empirical formula of a hydrocarbon is $CH _{3}.$ It's molecular weight is $30$, What is molecular formula of the compound.

  1. $CH _4$
  2. $C _2H _6$
  3. $C _3H _8$
  4. $C _4H _{10}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

  • Formula for finding molecular formula is
  • Molecular formula= n x Empirical formula
  • Now n = molecular weight /  empirical weight
  • therefore n = 30/15
        Where 15 is the weight of the empirical formula CH3
        n=2
Now the molecular formula = 2 x CH3
                                               = C2H6
Therefore the answer option B is correct.

Multiple choice urea organic compounds with functional group containing nitrogen

What is nitrogen content of urea?

  1. 54%

  2. 40%

  3. 46%

  4. 56%
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Urea, also known as carbamide, is an organic compound. This amide has two $-NH _2$ groups joined by a carbonyl ($C=O$) functional group. Urea can be applied to soil as a solid or solution or to certain crops as a foliar spray. Urea usage involves little or no fire or explosion hazard.

Urea's high analysis, 46% $N$, helps reduce handling, storage, and transportation costs over other dry $N$ forms.

Multiple choice urea organic compounds with functional group containing nitrogen

Percentage weight of hydrogen in urea is:

  1. 6.67%

  2. 10%

  3. 4%

  4. 12.33%
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Symbol Element Atomic Weight Atoms  Mass percent
 $C$  Carbon  12  1  19.99%
 $O$  Oxygen  16  1  26.64%
 $N$  Nitrogen  14  2  46.64%
 $H$  Hydrogen  1  4  6.67%

Molecular formula of Urea is ${ (N{ H } _{ 2 }) } _{ 2 }CO$.