List of top Questions asked in IISER- 2026

Consider the following sets of points in the complex plane \[ A = \left\{ \cos \left( \frac{2n\pi}{5} \right) + i \sin \left( \frac{2n\pi}{5} \right) : n \in \mathbb{Z} \right\} \text{ and} \] \[ B = \left\{ \cos \left( \frac{2n}{5} \right) + i \sin \left( \frac{2n}{5} \right) : n \in \mathbb{Z} \right\} . \] Which of the following statements is TRUE?

Let $p(x)$ be a quadratic polynomial such that $p(1) = p(-1) = 0$. What is the coefficient of $x$ in $p(x)$?

The rate constant of a reaction at 600 K with an activation energy of 191.47 kJ mol$^{-1}$ is 5.0 $\times$ 10$^{-5}$ s$^{-1}$. What is the temperature at which the half-life of the reaction becomes 152 s? [Consider pre-exponential factor and activation energy to be independent of temperature. R = 8.314 J K$^{-1}$mol$^{-1}$]}

Let $r, l$ be two integers such that $r \ge l \ge 3$. What is the total number of functions \[ f : \{1, 2, \dots, r\} \to \{1, 2, \dots, r\} \] such that $f(1), f(2), \dots, f(l)$ are all distinct?

Let $l_1$ be the line joining $(1, 1, 1)$ and $(3, 1, 3)$ and let $l_2$ be the line joining $(0, 2, -1)$ and $(2, 0, 3)$. What is the angle between $l_1$ and $l_2$?

Consider the points $A(4\hat{i} + \hat{j} + 3\hat{k})$, $B(2\hat{j})$ and $C(-4\hat{i} + 3\hat{j} - 3\hat{k})$. Which of the following statements is TRUE?

What are X and P in the following reaction sequence?

What is the order of pK$_b$ for the following molecules in an aqueous medium?

What is the ratio of the velocity of an electron in the fourth orbit of Be$^{3+}$ to the velocity of the electron in the second orbit of He$^{+}$?

For two pure volatile liquids X and Y, attractive intermolecular interactions of both X-X and Y-Y are weaker than those of X-Y. The total vapour pressure of an equimolar solution of X and Y is p$_{\text{total}$. The vapour pressure of pure X and pure Y are p$^0_{\text{X}}$ and p$^0_{\text{Y}}$, respectively. Which one of the following relations is correct?

An ideal gas goes through a reversible isothermal expansion (solid line) followed by a reversible adiabatic expansion (dashed line). Which of the following diagram(s) closely depict(s) the entire process?

100 mL of 1.0 M aqueous NaOH solution was diluted to 1.0 L by adding water. Half of this solution was discarded. A new 100 mL of 0.5 M aqueous NaOH solution was added to the remaining solution. What is the concentration of the final aqueous NaOH solution?

Which one is an INCORRECT statement with regard to the following reaction?

Which one of the following molecules shows an increase in bond order after loss of an electron from the highest occupied molecular orbital?

Metal-ligand $\pi$-bond formation in Mn$_2$(CO)$_{10}$ and [MnO$_4$]$^-$ requires electron-pair donation between metal and ligand orbitals. Which one of the following represents the direction of electron-pair donation?

What is the order of bond energy between C=S & C=Te, and between Cl–Cl & F–F?

Which one of the following molecules is chiral?

Consider the following silica-gel based thin-layer chromatogram of compounds A and B. Which one of the following statements is correct?

Which one of the following molecules can be used for RNA interference?

What are the numbers of protons (H$^+$) and electrons (e$^-$), respectively, required for the reduction of [Cr$_2$O$_7$]$^{2-}$ to Cr$^{3+}$ under an aqueous acidic condition?

A female child is born with all the primary oocytes required during her lifetime. At which one of the following stages of cell division are these oocytes found at birth?

The following pedigree diagram shows the inheritance of a rare genetic disorder in a family (filled shapes depict affected individuals). Which one of the following is the most likely pattern of inheritance of the disorder?

For which one of the following parents, their children will NOT have the same blood group phenotype as either of the parents?

Which one of the following octahedral complexes has the highest spin-only magnetic moment?

Which one of the following correctly describes the mode of action of Follicle Stimulating Hormone (FSH) and estrogen?