WAEC Chemistry Past Questions & Answers - Page 170

846.

Consider the reaction represented by the equation. 2NaHCO_3(s) \(\rightarrow\) 2Na₂CO _3(S) + CO _2(g) + H₂O_(g) what volume of carbon (IV)oxide at s.t.p is evolved when 0. 5 moles of NaHCO₃ is heated? [Molar volume = 22.4 dm³ at s.t. p]

1.12dm³

2.24dm³

5.6dm³

56.0dm³

View answer & explanation

Correct answer is C

2NaHCO_3(s) \(\rightarrow\) 2Na₂CO _3(S) + CO _2(g) + H₂O_(g)

2 moles of 2NaHCO_3(s) produces 22.4dm³ of CO₂

0.5 moles of 2NaHCO₃ will produce \(\frac{0.5 \times 22.4}{2}\)

= \(\frac{11.2}{2}\)

= 5.6 dm³

847.

If 20 cm3 of distilled water is added to 80cm 3 of 0. 50 mol dm-3 hydrochloric acid, the new concentration of the acid will be

0.10mol dm^-3

0.20mol dm^-3

0.40mol dm^-3

2.00mol dm^-3

View answer & explanation

Correct answer is C

\(V_1\) = 80cm3, \(C_1\) = 0.5mol/dm3

\(C_2\) = ? \(V_2) = 20cm3 + 80cm3 = 100cm3

using the dilution formula, \(C_1 V_1 = C_2 V_2\)

0.5 x 80 = \(C_2\) x 100

\(C_2 = \frac{ 40}{100}\) = 0.4mol\(dm^{-3}\)

848.

If 1 mole of sodium contains 6 x 10²³ atoms, how many atoms are contained in 0.6 g of sodium? [Na = 23]

1.56 x 10²³

1.56 x 10²²

3.6 x 10²³

3.6 x 10²²

View answer & explanation

Correct answer is B

23g of sodium contain 6 x 10₂₃ atoms :. 0.6g of sodium will contain \(\frac{0.6 x 6 x 10²³}{23}/) = 1.56 x 10²²

849.

An atom ²³⁸₉₂X decays by alpha particle emission to give an atom Y. The atomic number and mass number of Y are

90 and 234 respectively

91 and 238 respectively

92 and 236 respectively

93 and 238 respectively

View answer & explanation

Correct answer is A

238₉₂ X 234 ₉₀Y + ^{4 ₂ He Atomic number is 90 while mass number is 234}

850.

An element X has isotopic masses of 6 and 7. if the relative abundance is 1 to 12.5 respectively, what is the relative atomic mass of X?

6.0

6.1

6.9

7.0

View answer & explanation

Correct answer is C

Contribution by isotope 6 = 6 x 1 = 6
contribution by isotope 7 = 7 x 12.5 = 87.5
This contribution is made by 1 + 12.5 = 13.5 atoms
Relative atomic mass of = 87.5 + 6 = 93.5/13.5 = 6.9