Revision Chemistry Questions For University Freshers July 2020

University/High School Chemistry Tutorials







1.​​ Question on Chemical Kinetics:

If doubling the concentration of a reactant increases the rate of reaction four times, what is the order of reaction with respect to the reactant?



N/B;​​ Order of​​ reaction​​ is the power or exponent to which the reactant is raised in a rate equation. It is determined experimentally, and not stiochiometrically (or from the stiochiometrical equation). For example:

​​ If ​​ 




​​ (where x+y is the order of reaction)

N/B;​​ Rate Law:​​ the law states that the rate of reaction is directly proportional to reactant concentration with respect to the order of reaction; the mathematical expression of the rate law is known as​​ rate equation.​​ i.e

r[A]x ………………….. (1) rate law

r=k[A]x ………………….. (2) rate equation


from equa (2) ……..

k= rAxr1A1x=r2A2x

  ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​​​ 


but…A2=2A1doubling ​​​​ r2=4r1four times

r14r1=[A]12A1        14=12x       122=12x         x=2

the reaction is therefore a second order reaction since​​ x=2


2. Question on Chemical Equilibrium

  • Calculate the equilibrium concentration of NO2​​ in a​​ solution prepared by dissolving​​ 0.20mol​​ of N2O4​​ in 400ml of chloroform. For the reaction  ​​ ​​​​ N2O4g   2NO2gin chloroform, ​​  ​​ k=1.07×10-5

  • What will be the percentage decomposition of the original​​ N2O4?


  • keq=rate of backward reversereaction concentrationrate of forward reaction concentration ​​ ​​​​  ​​ (where​​ keq=equilibrium constant)​​ 


keq=1.07×10-5NO2=?                    N2O4=0.20mol400ml1000=0.5M


NO2=5.35×10-6=2.31×10-3 or 0.0023M


  • N2O4(g)        2NO2(g)

percentage composition = N2O4NO2×1001  =  0.462%


3. Question on Electromagnetic Radiation

  • What is an​​ electromagnetic radiation?

  • What is the wavelength (in nanometers) that corresponds to an electron transition from the​​ n=5 ​​​​ level to the​​ n=3​​ level in the hydrogen atom?​​ 


3a. Electromagnetic Radiation (EMR) is the propagation of energy in wave form, with periodic changes in magnetic and electric fields. The following are characteristics of EMR:

  • Wavelength (λ)

  • Velocity (speed of light [c])

  • Frequency (f or v)

  • Wavenumber (v-)

N/B:c=fλ c=velocity or speed of light,       f=frequency,              λ=wavelenght


1λ=R1n12-1n22                                        where;  n2>n1

λ=wavelength=?,          R=Rydberg's constant=10967758m-1,       n2=5,       n1=3

1λ=10967758132-152      =     1096775819-125       =        1096775825-99×25

1λ=10967758×16225   =   175484128225  =  779929.4578

λ=1779929.4578  =  1.2822×10-6m  =  1282.2nm



4. Question on Gas Laws​​ ​​ 

  • State Graham’s law of diffusion of gases. Express the equation in terms of rate, density, molar mass and time.

  • A mixture of two gases, A and B, of equal masses is made to diffuse through a porous pot. If it takes 30secs for A to diffuse through the pot, how long​​ will it take B?​​ [molar masses of A and B are 28 and 44 respectively]

  • Express the Van Der Waal’s equation for real gases.


  • Graham’s law of diffusion of gases states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its density at constant​​ temperature and pressure.​​ 


r1d ,          thus,   r=kd ,           therefore,  rd =k,       hence,  rArB=dBdA


The expression of the equation in terms of rate, density, molar mass and time follows as



  • MBMA=tBta          tB=tA×MBMA           tB=30×4428            tB=37.61secs


  • Expression of the Van Der Waal’s equation is also known as the real gas equation





5. Question on Acids/ Bases/ pH & pOH Analysis/ Nuclear Reactions​​ 

  • Water is said to be amphoteric. Illustrate this with a suitable example and equation

  • Prove that​​ pH=pOH=7

  • What are the differences between nuclear reactions and other chemical reactions?


  • An amphoteric​​ substance is one that has the ability to donate hydrogen ion (H+) or accept hydrogen ion (H+) in solution. E.g. water (H2O), ammonia (NH3) etc. They behave both like an acid and a base in solution.

H2O  H++OH-                                     



  • pH+pOH=14       

kw=ka+kb  ,                 kw=1.0×10-14

H2O  H++OH-

kw=H+OH-but when H+=OH-=x,

kw=x2      x=1.0×10-14 =1.0×10-7

H+=1.0×10-7      :pH=-logH+=7

OH-=1.0×10-7    :pOH=-logOH-=7



  • Differences between chemical and​​ nuclear reactions





Only the electronic structure of the atoms are affected

The nucleus of the atoms are affected​​ 


Does not result in formation of a new element

Results in conversion of one element to another


Are affected by factors that affect rate of chemical reactions such as temperature and pressure

Are not affected by factors that affect rate chemical reactions


Small amount of energy is released or absorbed by the reaction

Large amounts of energy is​​ released or absorbed by the reaction


Isotopes of an element show identical behaviour

Isotopes of an element behave differently​​ 



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