NECO Past Questions and Answers

961.

Determine the value of x from the nuclear reaction below,

\(^{222}_{86}Rn\) → \(^{x}_{88}Ra\) + 2β + energy

220

222

224

226

228

View answer & explanation

Correct answer is B

The equation above is Beta-decay:
where β is an electron → \(^{0} _{-1}e\)

2β in the equation = \(^{0} _{-1}e\) + \(^{0} _{-1}e\) → \(^{0} _{-2}e\)

\(^{222}_{86}Rn\) → \(^{x}_{88}Ra\) + \(^{0} _{-2}e\) + energy

where \(^{x}_{88}Ra\) + \(^{0} _{-2}e\) → \(^{222}_{86}Rn\)

: x = 222 for element Radon [ \(^{222}_{88}Ra\)

962.

Which of the following substances undergoes sublimation?

Ether

Ice

Iodine

Naphthalene

Turpentine

View answer & explanation

Correct answer is C

No explanation has been provided for this answer.

963.

The space between the two walls of a vacuum flask is completely evacuated to reduce heat loss by

condensation

conduction

convection

evaporation

radiation

View answer & explanation

Correct answer is C

No explanation has been provided for this answer.

964.

An observer heard the sound of thunder 5.8s after the lightning flash was seen. How far was he from the source? [Speed of sound in air = 330ms-1]

957m

1914m

3828m

8700m

17400m

View answer & explanation

Correct answer is B

VELOCITY=DISTANCE/TIME

Distance = 330×5.8 Distance = 1914m

965.

The wave theory of light does not explain photoelectricity because

light waves carry little energy

particles cannot be emitted by waves

the frequency of light waves too high

there is a threshold wavelength for electron emission

the wavelength of Light waves is too short

View answer & explanation

Correct answer is A

According to wave theory, after the light falls on a substance electrons are emitted after a small instant of time. However, in photoelectric effect, the electron emissions are immediate without a time delay.
The energy of a wave increases with increase in intensity. However, in photoelectirc effect, increase in intensity has no effect on the energy of electrons emitted. All that happens is the number of electrons emitted, increases.he energy of a wave grows as its intensity increases. In the photoelectronic effect, however, increasing the intensity has no influence on the energy of electrons released.