# 210+ Most Important Questions for JEE Main 2021

**Most Important Questions for JEE Main 2021**: In this article, we are providing 210+ most important questions with answers and explanations from all the three subjects that are Physics, Chemistry and Mathematics for JEE Main 2021.

*Most Important Questions for JEE Main 2020*

**Important Questions for JEE Main 2021: **As part of the JEE Main preparation, students must have gone through different coaching institutes, online websites and test series. Students take help from these resources in their preparation for JEE Main 2021. We all know the value of guess papers just days before JEE Exam date. We know that engineering entrance exams like JEE Main, JEE Advanced, UPSEE, WBJEE, VITEEE and SRMJEEE are primarily based on the class 11th and class 12th syllabus which is huge. So, for all these exams we have identified 210+ questions with answers and explanations from all the three subjects, i.e., Physics, Chemistry and Mathematics. Students must go through all the questions listed below to revise the important topics by practising the questions. These questions will help you to score good marks and a good rank in the engineering entrance exams especially in JEE Main 2021.

**JEE Main Admit Card 2021 Download Link: Exam Date - 23, 24, 25 & 26 February 2021**

**JEE Main 2021 & NEET 2021: Check Important Chemical Reactions from CBSE Syllabus & NCERT Textbooks**

**Few sample questions are given below:**

**Question:**

Select the correct stability order of the conformation /Rotamers/ Rotational isomers of Ethane Molecule

(a) Partially Eclipsed > Fully Eclipsed > Anti-staggered > Skew or Gouche

(b) Skew or Gouche > Partially Eclipsed > Fully Eclipsed > Anti-staggered

(c) Anti-staggered > Skew or Gouche > Partially Eclipsed > Fully Eclipsed

(d) Fully Eclipsed > Anti-staggered > Skew or Gouche > Partially Eclipsed

**Solution:**

The isomers which can be produced by the free rotation about single bond are known as conformation /Rotamers/ Rotational isomers and this phenomenon is called as conformational analysis

These isomers can be represented by

1. Sawhorse projection

2. Newman’s projection formula

3. A staggered conformer is more stable than an eclipsed conformer as the latter involves unfavourable energy interactions between atoms.

4. In conformational analysis (energetics study of different rotational conformers or rotamers), structures are represented by *Newman projection * (atoms and bonds are viewed along the axis of rotation).

Hence, the correct answer is (c).

**Question:**

As per IUPAC nomenclature, the name of the complex [Co (H_{2}O)_{4}(NH_{3} )_{2}]C1_{3} is

(a) tetraaquadiaminecobalt (III) chloride

(b) tetraaquadiamminecobalt (III) chloride

(c) diaminetetraaquacobalt (III) chloride

(d) diamminetctraaquacobalt (III) chloride

**Solution:**

First of all, the compound has complex positive part [Co(H_{2}O)_{4}(NH_{3})_{2}]^{3+} therefore, according to IUPAC conventions, positive part will be named first. Secondly, in writing name of complex, ligands are named first in alphabetical order, irrespective of its charge, hence "ammine" will be written prior to "aqua".

**NOTE:** In alphabetical order, original name of ligands are considered not the initials of prefixes. Also, special precaution should be taken in spelling name of NH_{3} ligand as it is ammine. Therefore, name of the complex is [Co(H_{2}O)_{4}(NH_{3})_{2}]Cl_{3} is Diamminetetraaqua cobalt (III) chloride.

Hence, the correct answer is (d).

**Question:**

Two numbers are chosen from 1, 3, 5, 7,… 147, 149 and 151 and multiplied together in all possible ways. The number of ways which will give us the product a multiple of 5, is

Options:

(a) 75

(b) 1030

(c) 95

(d) 1020

**Solution:**

In the given numbers 1, 3,5,7,…, 147, 148, 151the numbers which are multiple of 5 are 5, 15, 25, 35,…, 145 which are an arithmetic sequence

Hence, the correct answer is (d).

**Question:**

**Solution:**

Hence, the correct answer is (d).

**Question:**

The image of an object, formed by a plano-convex lens at a distance of 8m behind the lens, is real and is one third the size of the object. The wavelength of light inside the lens is 2/3 times the wavelength in free space. The radius of the curved surface of the lens is

(a) 1 m

(b) 6 m

(c) 5 m

(d) 3 m

**Solution:**

Hence, the correct answer is (d).

**Question:**

The resistance of a wire at room temperature 20°C is found to be 20W. Now to increase the resistance by 20%, the temperature of the wire must be [The temperature coefficient of resistance of the material of the wire is 0.002 per °C].

Options:

(a) 142°C

(b) 124°C

(c) 150°C

(d) 122°C

**Solution:**

Hence, the correct answer is (b).

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