What are significant figures? State rules to determine it.

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$2\text{N}_2\text{O(g)}\rightleftharpoons2\text{N}_2(\text{g})+\text{O}_2\text{(g)}$
$2\text{NH}_3\text{(g)}\rightleftharpoons\text{N}_2(\text{g})+3\text{H}_2\text{(g)}$
$2\text{Cu}(\text{NO}_3)_2(\text{s})\rightleftharpoons2\text{CuO(s)}+4\text{NO}_2(\text{g})+\text{O}_2(\text{g})$
$\text{CH}_3\text{COOC}_2\text{H}_5(\text{aq})+\text{H}_2\text{O}\text{l}\\\rightleftharpoons\text{CH}_3\text{COOH(aq)}+\text{C}_2\text{H}_5\text{OH(aq)}$
$\text{Fe}^{3+}(\text{aq})+\text{3OH}^-\text{(aq)}\rightleftharpoons\text{Fe(OH)}_3(\text{s})$

Why is +2 oxidation state of Mn (25) quite stable, while the same is not true for iron (26)?
What is meant by dual nature of electrons? Calculate the energy and wavelength of the photon emitted by hydrogen atom when electron makes a transition from n = 2 to n = 1. Given that the ionisation potential is 13.6 eV. (1eV = 1.6 × 10^-19J).

The density of the water at room temperature is 0.1g/ mL. How many molecules are there in a drop of water if its volume is 0.05mL?
An alloy of iron (53.6%), nickel (45.8%) and manganese (0.6%) has a density of 8.17g cm^-3

Calculate the number of Ni atoms present in the alloy of dimensions 10.0cm × 20.0cm × 15.0cm

Define solubility product. Write solubility product expression for Zr₃(PO₄)₄.
Calculate the pH of 0.01 M CH₃COOH solution. [K_a(CH₂COOH) = 1.74 × 10^-51
Explain why NaCl is precipitated when HCl(g) is passed through the saturated solution of NaCl.