Math & Science ⇒ referencing a non-existing label in equation

[kalmer]

i use align as in the following example:

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\begin{align}
RH :&= \cfrac{m_w}{m_\text{sat}(w)} = \cfrac{M_w \cdot n_w}{M_\text{sat}(w) \cdot n_\text{sat}(w)} = \cfrac{M_w \cdot \frac{p_w \cdot \cancel{V}}{\cancel{R \cdot T}}}{M_\text{sat}(w) \cdot \cfrac{p_\text{sat}(w) \cdot \cancel{V}}{\cancel{R \cdot T}}} = \frac{p_w}{p_\text{sat}(w)}\\
&= \frac{\frac{W \times p}{W + `r M_w_TO_M_d`}}{p_\text{sat}(w)} = \frac{W \times p}{(W + `r M_w_TO_M_d`) \cdot p_\text{sat}(w)} = \frac{x_w}{x_\text{sat}(w)}|_{T, P}
(\#eq:relative-humidity)
\end{align}

Based on this source code, a multi-line equation will be created having two numeric labels on the right-hand side, one for each line. If i want to reference the the equation, i use \@ref(eq:relative-humidity) and that works fine. However, i want to refer the first line, not the second line. How can i do that? i tried the following:

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\begin{align}
RH :&= \cfrac{m_w}{m_\text{sat}(w)} = \cfrac{M_w \cdot n_w}{M_\text{sat}(w) \cdot n_\text{sat}(w)} = \cfrac{M_w \cdot \frac{p_w \cdot \cancel{V}}{\cancel{R \cdot T}}}{M_\text{sat}(w) \cdot \cfrac{p_\text{sat}(w) \cdot \cancel{V}}{\cancel{R \cdot T}}} = \frac{p_w}{p_\text{sat}(w)}\\(\#eq:relative-humidity-ratio-of-pressures)
&= \frac{\frac{W \times p}{W + `r M_w_TO_M_d`}}{p_\text{sat}(w)} = \frac{W \times p}{(W + `r M_w_TO_M_d`) \cdot p_\text{sat}(w)} = \frac{x_w}{x_\text{sat}(w)}|_{T, P}
(\#eq:relative-humidity)
\end{align}

That resulted with:

! Package amsmath Error: Multiple \label's: label 'eq:relative-humidity-ratio-of-pressures' will be lost.

[Bartman]

Please expand your example to a complete minimal working example that produces your error message.

A line in the align environment can only be referenced if one \label command precedes the end of the line. In your case the first line would have no label and the second two or multiple, as the error message states.

[Kreuz Elf]

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(\#eq:relative-humidity-ratio-of-pressures)

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(\#eq:relative-humidity)

I've not seen this syntax before. Maybe it is a legit variant, but consider using \lable{eq:relative-humidity-ratio-of-pressures}