NEET Chemistry Questions: Some Basic Concepts in Chemistry

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In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A : Atomic mass of sodium is 23 u. R : An atom of sodium is 23 times heavier than an atom of $ {12} ^ C $




No explanation available.
In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A: Pure water, irrespective of its source always contain hydrogen and oxygen in the ratio 1 : 8 bymass. R : Total mass of reactants and products remains constant during physical or chemical change




The statement A is correct as pure water always contains hydrogen and oxygen in the ratio of 1:8 by mass. The statement R is also correct as the total mass of reactants and products remains constant during a chemical reaction. However, R is not the correct explanation for A.
In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A : Mass numbers of most of the elements are fractional. R: Mass numbers are obtained by comparing with mass number of $ {12} ^ C $ .




Both statements A and R are incorrect. Mass numbers of elements are always whole numbers and not fractional. Mass numbers are determined by comparing with the mass of a carbon-12 atom, which is assigned a mass number of 12 exactly.
In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A : The mass of the products formed in a reaction depends upon the limting reactant. R: Limting reactant reacts completely in the reaction.




The mass of products formed in a reaction depends on the limiting reactant because the limiting reactant gets consumed completely. The other reactants remain in excess.
In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A : Cinnabar is a chemical compound whereas brass is mixture. R : Cinnabar always contain 6.25 times mercury than sulplur by weight. Brass can have any proportion of Cu and Zn.




Cinnabar is a chemical compound (HgS) with a fixed composition, while brass is an alloy (mixture) of copper and zinc in varying proportions. The statement about cinnabar's composition is correct.
In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A : $1 L of O_2 and 1 L of O_3 $ contains the same number of moles under identical conditions. R : Under identical conditions, $1 L of O_2 and 1 L of O_3 $ contain the same number of oxygen atoms




The assertion that 1L of O2 and 1L of O3 contain the same number of moles under identical conditions is correct, as the mole is a measure of the amount of substance. However, the reason is incorrect because O3 has more oxygen atoms per molecule compared to O2.
In each of the follwoing questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below : A : The standard unit for expressing atomic mass is amu. R : Now a days amu is represented by ‘u’.




The assertion that the standard unit for expressing atomic mass is amu (atomic mass unit) is correct. The reason that nowadays amu is represented by 'u' is also correct, but it does not explain the assertion. The unit 'u' is simply a modern representation of the same atomic mass unit.

How many grams of concentrated nitric acid solution should be used to prepare 250 mL of 2.0 M HNO3? The concentrated acid is 70% HNO3.





(a) Given, molarity of solution=2

Volume of solution = 250mL=250/1000=1/4L

Molar mass of HNO3 = 1+14+3x16=63g mol-1

Molarity = weight of HNO3Molecular mass  of HNO3×volume of solution(L)

Weight of  HNO3 = molarity x molecular massx volume(L) = 2x63x1/4g=31.5g

It is the weight of 100%  HNO3

But the given acid is 70% HNO3

Its weight = 31.5x100/70g=45g

 

Mole fraction of the solute in a 1.00 molal aqueous solution is 





(a) 1.00 molal aqueous solution = 1.0 mole in 1000g water

nsolute = 1; Wsolvent =1000g

nsolvent = 1000/18 = 55.56

Xsolute = nsolutensolute+nsolvent

Xsolute = 1/1+55.56 = 0.0177 

Alternate Method

Molality(m) = 1000xn/NxM

where, n = number of moles of solute

         N = number of moles of solvent

        M = molar mass of solvent

Given, m=1

... 1= 1000xn/Nx18 ⇒n/N =18/1000

or n/n+N = 18/1018 =0.0177

6.02x1020 molecules of urea are present in 100mL of its solution. The concentration of solution is 





(b) Given, number of molecules of urea= 6.02x1020 

Number of moles = 6.02x1020 /NA

=6.02x1020 /6.02x1023 =1x10-3 mol

Volume of the solution = 100mL = 100/1000L =0.1L

Concentration of urea solution (in mol L-1)

=1x10-3 /0.1 mol L-1

=1x10-2 mol L-1

=0.01 mol L-1

 

Which has maximum number of molecules?





(b) In 7g nitrogen, number of  molecules = 7.028mol = 0.25 x NA molecules

where, NA = Avogadro number = 6.023 x 1023

In 2g of H2= 2/2 mol =1x NA molecules

In 16 g of  NO2= 16/46 mol = 0.348 x NA molecules

In 16 g of O2 = 16/32 mol = 0.5 x NA molecules

Hence, maximum number of molecules are present in 2g of H2.

The number of moles of oxygen in 1L of air containing 21% oxygen by volume, under standard conditions, is 





(a) Volume oxygen in 1L of air = 21/100x1000 = 210mL

22400 mL volume at STP is occupied by oxygen = 1mole

Therefore, number of moles occupied by 210mL = 210/22400 = 0.0093mol

What volume of  oxygen gas (O2) measured at 0°C and 1 atm, is needed to burn completely 1L of  propane gas (C3H8) measured under the same conditions?





(c) C3H8 + 5O2  →3CO2+ 4H2O

22.4L   5x22.4L

For the combustion of 22.4L propane, oxygen required = 5 x 22.4L

For  the combustion of 1L of propane oxygen required = 5x22.4/22.4L

=5L

Percentage of Se in peroxidase anhydrase enzyme is 0.5% by weight (at. weight = 78.4), then minimum molecular weight of peroxidase anhydrase enzyme is





(d) Suppose the molecular weight of enzyme =  x

0.5% by weight means in 100g of enzyme weight of Se=0.5g

In xg of enzyme weight of Se = .5100×x

Hence, 78.4 = 0.5 × x100

x = 15680 = 1.568x104

Number of moles of MnO4- required to oxidise one mole of ferrous oxalate completely in acidic medium will be 





0.6

10FeC2O4 + 6KMnO4 + 24H2SO4 ----> 3K2SO4+ 6MnSO4 +  5Fe2(SO4)3 + 24H2O + 20CO2

So we see that  6 moles of KMnO4 is required to oxidize 10 moles of  FeC2O4  Then, 1 mole of FeC2O4 would be oxidized by = ?    6/10 = 0.6