A sample of boron was found to have the following % composition by mass: B 5 10 18 . 7 % , B 5 11 81 . 3 % . Calculate a value for the relative atomic mass of boron. Give your answer to 3 significant figures.

Relative atomic mass is defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant. The atomic mass constant is defined as being 1 12 of the mass of a carbon - 12 atom. Relative atomic mass = The sum of products of isotopic masses with their percentages of an element 100 Then, the relative atomic mass of boron = 10 . 0 × 18 . 7 + 11 . 0 × 81 . 3 100 = 10 . 813 amu .

The mass spectrum of zirconium is shown below. Give the isotopic symbol for the most abundant isotope of zirconium.

Isotopes are defined as one of two or more chemical species of atoms of an element with the same atomic number but with different atomic masses due to the presence different neutron numbers. Zirconium has five naturally occurring isotopes. 90 Zr , 91 Zr , 92 Zr , 94 Zr and 96 Zr (the longest-lived radioisotope of zirconium). Zr 40 90 is the most common, making up 51 . 50 % of all zirconium.

A sample of 15 . 2 g of tin(IV) oxide is mixed with 2 . 41 g of carbon and heated. A reaction occurs. SnO 2 + 2 C → Sn + 2 CO Show by calculation that the reagent in excess is tin(IV) oxide. ( A r values: Sn = 118 . 7 , C = 12 . 0 , O = 16 . 0 ).

In a chemical reaction, reactants that are not used up when the reaction is finished are called excess reagents. To find the excess reagent we have to calculate the moles number of each reactant for the given reaction by using stoichiometric reaction, SnO 2 + 2 C → Sn + 2 CO From the above reaction it is confirmed that 1 mole of tin IV oxide reacts with 2 moles of carbon to give 1 mole of tin and 2 moles of carbon monoxide. In the problem they have given, Weight of tin IV oxide = 15 . 2 g Molar mass of tin IV oxide = 150 . 7 g / mol Then, moles number of tin IV oxide = 15 . 2 150 . 7 = 0 . 1009 mol Now we will calculate moles of carbon required for complete consumption of 0 . 1009 mol tin IV oxide by using conversion factor, = 0 . 1009 mol × 2 mol of C 1 mol of SnO 2 = 0 . 1009 × 2 1 = 0 . 2018 mol of carbon needed. Weight of carbon = 2 . 41 g Then, moles number of carbon = 2 . 41 12 . 0 = 0 . 2008 mol of carbon reacted So, from the above values for 0 . 2018 mol of carbon only 0 . 2008 mol carbon is added, this indicates SnO 2 is excess reagent.

A sample of zirconium is made by reacting 58 . 30 g of zirconium tetrachloride, ZrCl 4 , with excess magnesium. ZrCl 4 + 2 Mg → Zr + 2 MgCl 2 The mass of zirconium produced is 20 . 52 g . Calculate the percentage yield of zirconium. ( A r values: Zr = 91 . 2 , Cl = 35 . 5 , Mg = 24 . 3 ).

Given, actual mass of zirconium produced is 20 . 52 g Theoretical mass of zirconium is 91 . 2 g Percentage yield = Actual yield Theoretical yield × 100 % Percentage yield = 20 . 52 91 . 2 = 0 . 225 % .

Solid sodium carbonate reacts with aqueous hydrochloric acid to form aqueous sodium chloride, carbon dioxide and water. Na 2 CO 3 + 2 HCl → 2 NaCl + CO 2 + H 2 O Calculate the number of moles of hydrochloric acid required to react exactly with 4 . 15 g of sodium carbonate. ( A r values: C = 12 . 0 , Na = 23 . 0 , O = 16 . 0 , H = 1 . 0 , Cl = 35 . 5 ).

Solid sodium carbonate reacts with aqueous hydrochloric acid to form aqueous sodium chloride, carbon dioxide and water. Na 2 CO 3 + 2 HCl → 2 NaCl + CO 2 + H 2 O Molar mas of sodium carbonate, Na 2 CO 3 = 2 × 23 + 1 × 12 + 3 × 16 = 106 g / mol . Based on stoichiometric equation, 106 g of sodium carbonate reacts with 2 moles of hydrochloric acid. 4 . 15 g of sodium carbonate reacts with = 4 . 15 × 2 106 = 0 . 078 moles of hydrochloric acid.

EASY

AS and A Level

IMPORTANT

Earn 100

State the meaning of the term relative atomic mass.

Important Questions on Atoms, Molecules and Stoichiometry

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 1

A sample of boron was found to have the following

%

composition by mass:

{}_{5}^{10}B (18.7 %)

{}_{5}^{11}B (81.3 %)

. Calculate a value for the relative atomic mass of boron. Give your answer to

3

significant figures.

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 1

Boron ions,

B^{3 +}

, can be formed by bombarding gaseous boron with high-energy electrons. Deduce the number of electrons in one

B^{3 +}

ion.

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 1

Boron is present in compounds called borates. The accurate relative atomic mass of iron, $Fe$ , is $55.8$ . Explain why the accurate relative atomic mass is not a whole number.

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 2

The mass spectrum of zirconium is shown below.

Question Image

Give the isotopic symbol for the most abundant isotope of zirconium.

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 2

A sample of $15.2 g$ of tin(IV) oxide is mixed with $2.41 g$ of carbon and heated. A reaction occurs.

${SnO}_{2} + 2 C \to Sn + 2 CO$

Show by calculation that the reagent in excess is tin(IV) oxide. ( $A_{r}$ values: $Sn = 118.7, C = 12.0, O = 16.0$ ).

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 2

A sample of zirconium is made by reacting $58.30 g$ of zirconium tetrachloride, ${ZrCl}_{4}$ , with excess magnesium.

${ZrCl}_{4} + 2 Mg \to Zr + 2 {MgCl}_{2}$

The mass of zirconium produced is $20.52 g$ . Calculate the percentage yield of zirconium. ( $A_{r}$ values: $Zr = 91.2, Cl = 35.5, Mg = 24.3$ ).

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 3

Solid sodium carbonate reacts with aqueous hydrochloric acid to form aqueous sodium chloride, carbon dioxide and water.

${Na}_{2} {CO}_{3} + 2 HCl \to 2 NaCl + {CO}_{2} + H_{2} O$

Calculate the number of moles of hydrochloric acid required to react exactly with $4.15 g$ of sodium carbonate.

( $A_{r}$ values: $C = 12.0, Na = 23.0, O = 16.0, H = 1.0, Cl = 35.5$ ).

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 3 - Atoms, Molecules and Stoichiometry>EXAM-STYLE QUESTIONS>Q 3

Define the term mole.

State the meaning of the term relative atomic mass.

Important Questions on Atoms, Molecules and Stoichiometry

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