# Aligning equations left and on one character

To establish a formula in chemistry, I would like everything to be aligned to the left. For the last three, I would like them to be aligned with the "=" sign : most of my equations are centered, usually I use \begin{aligned} but not possible here as I want left alignment .

How can I do ?

Thanks

\documentclass{article}
\usepackage{graphicx} % Required for inserting images
\usepackage{amsmath}

\title{equation}
\author{nlbmoi }
\date{April 2024}

\begin{document}
On peut alors réécrire la constante d'acidité $K_a$ en fonction de la concentration initiale $C_0$ d'acide et le coefficient de dissociation $\alpha$ :
$$\alpha = \frac{[A^-]}{C_0} \\ \text{ on en déduit : } [A^-]=\alpha \cdot C_0 \\ \text{la conservation de l'espèce s'écruit : } [HA] + [A^-] = C_0\\ \text{d'où la relation : } [HA]=C_0 -[A^-] = C_0 (1-\alpha)\\ \text{on peut montrer (par un tableau d'avancement par exemple) que } [A^-]=[H_3O^+]\\ K_a &=\frac{[H_3O^+][A^-]}{[HA]} &=\frac{\alpha^2 \cdot {C_0}^2}{C_0 (1-\alpha)}\\ & =C_0\frac{\alpha^2}{1-\alpha} \label{eq:ka_coef_dissociation}$$
\end{document}

• 1. \\ does nothing at all inside equation. 2. & cannot be used in equation. It's quite hard to guess what your intentions are, sorry. Commented Apr 19 at 21:57
• Without further information, I'd say this is a duplicate of Align equation left - TeX - LaTeX Stack Exchange . (note that there are two possible interpretations of left align, read my answer there.) Commented Apr 19 at 22:40

## 1 Answer

I use \begin{aligned} but not possible here as I want left alignment .

That's not correct. In fact, I suggest you employ nested aligned environments. In addition, I'd also employ a package such as mhchem and its \ce macro to help simplify and streamline the typesetting of chemical expressions.

\documentclass{article}
\usepackage{amsmath}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{mhchem} % to typeset chemical formulas

\begin{document}
On peut alors réécrire la constante d'acidité $K_a$ en fonction de la concentration initiale $C_0$ d'acide et le coefficient de dissociation $\alpha$:
\label{eq:ka_coef_dissociation} \begin{aligned} &\alpha = \frac{\ce{[A^-]}}{C_0} \\ &\text{on en déduit: } \ce{[A^-]}=\alpha \cdot C_0 \\ &\text{la conservation de l'espèce s'écruit: } \ce{[HA] + [A^-]} = C_0\\ &\text{d'où la relation: } \ce{[HA]}=C_0 -\ce{[A^-]} = C_0 (1-\alpha)\\ &\text{on peut montrer (par un tableau d'avancement par exemple)}\\ &\quad\text{que } \ce{[A^-]}=\ce{[H3O^+]}\\ &\begin{aligned} K_a &=\frac{\ce{[H3O^+][A^-]}}{\ce{[HA]}} \\ &=\frac{\alpha^2 \cdot {C_0}^2}{C_0 (1-\alpha)}\\ &=\alpha^2/(1-\alpha) C_0 \end{aligned} \end{aligned}
\end{document}