# In each of the following pairs, which one has the lower viscosity?

## a. $\text{HCOOH}$ or ${\text{C"_8"H}}_{18}$ b. $\text{C"_2"H"_5"Cl}$ or $\text{CH"_3"OH}$

Jul 31, 2015

a. $\text{C"_8"H"_18}$ and b. $\text{C"_2"H"_5"Cl}$ have the lower viscosities.

#### Explanation:

The viscosity of a liquid is determined by its intermolecular forces.

The stronger the intermolecular forces, the more difficult it is for the molecules to slide past each other.

a. $\text{HCOOH}$ or ${\text{C"_8"H}}_{18}$

${\text{C"_8"H}}_{18}$ has only weak London dispersion forces, while $\text{HCOOH}$ has strong intermolecular hydrogen bonds.

So we predict that ${\text{C"_8"H}}_{18}$ has a lower viscosity than $\text{HCOOH}$.

The prediction is confirmed by the actual viscosities.

The viscosity of ${\text{C"_8"H}}_{18}$ is 0.542 mPa·s, while the viscosity of $\text{CH"_3"-OH}$ is 1.78 mPa·s.

b. $\text{C"_2"H"_5"Cl}$ or $\text{CH"_3"-OH}$

$\text{C"_2"H"_5"Cl}$ or $\text{CH"_3"-OH}$

$\text{C"_2"H"_5"Cl}$ has weak dipole-dipole forces from the $\text{C-Cl}$ bonds, but $\text{CH"_3"-OH}$ has strong hydrogen bonds.

Confirmation: The viscosity of $\text{C"_2"H"_5"Cl}$ is 0.279 mPa·s, while the viscosity of $\text{CH"_3"-OH}$ is 0.593 mPa·s.