# Formulas You’ll Need to Know for the HESI® Chemistry Test

Studying for the Health Education Systems, Inc. (HESI) exam can be challenging, particularly when it comes to the chemistry section. This test, widely used in the United States for nursing school admissions, covers a wide range of topics. This article is a brief guide on some of the essential formulas you’ll need to understand and apply for the chemistry portion of the HESI exam. Let’s delve in.

## Density

One key concept you’ll need to grasp is density, which is a measure of how much mass is packed into a given volume. The formula for density is:

$d = \frac{m}{v}$

where:

• d is density (usually in g/cm³)

• m is mass (usually in grams)

• v is volume (usually in cm³)

Knowing how to calculate and interpret density can help you understand many different chemical properties and phenomena.

## Atomic Structure

Another important area you’ll need to understand is atomic structure. The structure of an atom is represented with a specific notation:

$^{A}_{Z}X$

where:

• A is the Mass Number (total number of protons and neutrons)

• Z is the Atomic Number (number of protons)

• X is the Atomic Symbol

The Mass Number A can be calculated by adding the number of protons Z and the number of neutrons N in an atom:

$A = Z + N$

The concept of the mole and Avogadro’s number is fundamental to chemistry. A mole is a unit of measure in chemistry that denotes an amount of a substance.

where:

$n \cdot N_a = a$
• n is the number of moles

• $$N_a = 6.022 \times 10^{23}$$ is Avogadro’s Number

• a is the number of atoms

This formula is essential when calculating quantities in chemical reactions and interpreting molecular and atomic structures.

## Molar Mass

Molar mass M is the mass of one mole of a substance, typically expressed in grams per mole (g/mole). It can be calculated as:

$M = \frac{m}{n}$

where:

• M is the molar mass (g/mole)

• m is mass (g)

• n is the number of moles

## Conservation of Mass

The Law of Conservation of Mass states that the total mass of the reactants in a chemical reaction is equal to the total mass of the products. This can be expressed as:

$m_r = m_p$

​ where: ​

• $$m_r$$ is the total mass of reactants (g)

• $$m_p$$ is the total mass of products (g)

## pH and pOH

The pH scale is used to measure the acidity or alkalinity of a solution. It can be calculated as:

$pH = - \log[H_3O^+]$

where:

$$pH$$ is the decimal cologarithm of Hydrogen

$$[H_3O^+]$$ is the Hydronium concentration (mole/L)

It’s important to understand the relationship between pH and pOH, which measures the concentration of hydroxide ions in a solution. The two are related by the formula:

$14 = pH + pOH$

where:

$$pH$$ is the decimal cologarithm of Hydrogen

$$pOH$$ is the decimal cologarithm of Hydroxide

These concepts are essential in understanding the properties of acids and bases and how they react in various chemical reactions.

## HESI Chemistry Formula Chart

This chart breaks down key topics into categories, formulas, and their corresponding symbols, providing a quick reference for your study sessions. The included categories span from basic concepts such as density and atomic structure to more complex topics involving moles, Avogadro’s number, molar mass, conservation of mass, and understanding of $$pH$$ and $$pOH$$ values.

$\begin{array}{|c|c|c|} \hline \textbf{Category} & \textbf{Formula} & \textbf{Symbols} \\ \hline & & d = \text{ density } (g/cm^3) \\ \text{Chemistry} & d = \dfrac{m}{v} & m = \text{ mass } (g) \\ & & v = \text{ volume } (cm^3)\\ \hline & & A = \text{ Mass Number } \\ \text{Chemistry} & ^{A}_{Z}X & Z = \text{ Atomic Number } = \text{ Number of Protons } \\ & & X = \text{ Atomic Symbol } \\ \hline & & A = \text{ Mass Number } \\ \text{Chemistry} & A = Z + N & Z = \text{ Atomic Number } = \text{ Number of Protons } \\ & & N = \text{ Number of Neutrons } \\ \hline & & n = \text{ number of moles } \\ \text{Chemistry} & n \cdot N_a = a & N_a = \text{ Avogadro's Number } = 6.022 \cdot 10^{23} (atoms/mole)\\ & & a = \text{ number of atoms } \\ \hline & & M = \text { molar mass } (g/mole) \\ \text{Chemistry} & M = \dfrac{m}{n} & m = \text{ mass } (g)\\ & & n = \text{ number of moles }\\ \hline \text{Chemistry} & m_r = m_p & m_r = \text{ total mass of reactants } (g) \\ & & m_p = \text{ total mass of products } (g) \\ \hline \text{Chemistry} & pH = - \log[H_3O^+] & pH = \text{ decimal cologarithm of Hydrogen } \\ & & [H_3O^+] = \text{ Hydronium conc. } (mole/L) \\ \hline \text{Chemistry} & 14 = pH + pOH & pH = \text{ decimal cologarithm of Hydrogen } \\ & & pOH = \text{ decimal cologarithm of Hydroxide } \\ \hline \end{array}$

## Get Ready to ACE the HESI Chemistry Section!

Understanding and applying these formulas is the key to mastering the chemistry portion of the test. Alongside this guide, make sure to invest time in practice tests and comprehensive study guides. These resources provide a more detailed overview of the Chemistry exam content, offering practice problems that can help you apply these formulas in realistic exam scenarios.

Studying for the HESI exam requires discipline and strategic planning. Make the best use of this formula guide, practice tests, and study guides, and you will be well-prepared to tackle the chemistry portion of the exam with confidence. Remember, the goal is to comprehend, not just to memorize. By understanding these fundamental concepts and formulas, you will not only ace your HESI exam but also lay a solid foundation for your future nursing career. Good luck!