2019 - JAMB Chemistry Past Questions and Answers - page 4
At 27°C, 58.5g of sodium chloride is present in 250cm\(^3\) of a solution. The solubility of sodium chloride at this temperature is?
(molar mass of sodium chloride = 111.0gmol\(^{-1}\))
Given the Mass of the salt = 58.5g
Volume = 250 cm\(^3\) = 0.25 dm\(^3\)
Mass concentration = \(\frac{Mass}{Volume}\)
= \(\frac{58.5}{0.25}\) = 234 gdm\(^{-3}\)
Solubility (in moldm\(^{-3}\) = \(\frac{234}{111} \)
= 2.11 moldm\(^{-3}\)
\(\approxeq\) 2.0 moldm\(^{-3}\)
Which of the following properties increases from left to right along the period but decreases down the group in the Periodic Table?
i. Atomic Number ii. Ionization energy iii. Metallic character iv. Electron affinity
Ionization energy and electron affinity increase across a period, and decrease down a group.
The molecular shape and bond angle of water are respectively
The shape of water molecule = Bent/ V- shaped
The bond angle of water = 104.5°/ 105°
Na\(_2\)CO\(_3\) + 2HCl \(\to\) 2NaCl + H\(_2\)O + CO\(_2\)
The indicator most suitable for this reaction should have a pH equal to
Methyl orange is the best indicator for the reaction with range 3.1 - 4.4.
A radioactive nucleus has a half-life of 20 years, starting with 100,000 particles, how many particles will be left exactly at the end of 40 years
75,000 particles
35,000 particles
25,000 particles
50,000 particles
Given:
t1/2 = 20 years
After the first 20 years, half of the substance (\(\frac{1}{2} \times 100,000 = 50,000\)) will have decayed. Hence, we have 100,000 - 50,000 = 50,000 particles left.
After the second 20 years (being 40 years in all), half of the remaining substance (\(\frac{1}{2} \times 50,000 = 25,000\)) will have decayed.
Remaining particles after 40 years = 50,000 - 25,000
= 25,000 particles.
Which two gases can be used for the demonstration of the fountain experiment?
Which of the following pollutants will lead to the depletion of ozone layer?
Which of the following describes the chemical property of acids?
Which of the following pairs cannot be represented with a chemical formula?
Consider the equation below:
Cr\(_2\)O\(_7 ^{2-}\) + 6Fe\(^{2+}\) + 14H\(^{+}\) \(\to\) 2Cr\(^{3+}\) + 6Fe\(^{3+}\) + 7H\(_2\)O.
The oxidation number of chromium changes from
+5 to +3
+6 to +3
-2 to +3
+7 to +3
Cr\(_2\)O\(_7 ^{2-}\) + 6Fe\(^{2+}\) + 14H\(^{+}\) \(\to\) 2Cr\(^{3+}\) + 6Fe\(^{3+}\) + 7H\(_2\)O
Oxidation of Cr in Cr\(_2\)O\(_7 ^{2-}\) :
Let oxidation of Cr = x
then,
2x + (-2 x 7) = -2 \(\implies\) 2x - 14 = -2
2x = 12 ; x = +6
Therefore,
the change in the oxidation of Cr = +6 to +3