How many significant figures should I report calculating volume concentrations? Let's say a $\pu{300mL}$ solution of $\ce{CuCl_2}$ was diluted to $\pu{900mL}$. I am then asked to calculate the concentration of the new solution. My solution to the problem is as follows:

$$\begin{align}M_1 \times V_1 &amp;= M_2 \times V_2\\[0.5em]
\pu{300mL} \times \pu{4M} &amp;= M_2 \times \pu{900mL}\\[0.5em]
M_2 &amp;= \pu{1.33M}\end{align}$$

Now I'm wondering how many significant figures my final answer should have. From what I know, I have to refer to the least significant measurement, which happens to have one significant figure. Should my answer be 1 instead?
We cannot say how many significant figures you should be reporting because:


We cannot be sure about the precision of your volume measurements; and
We cannot be sure about the precision of the original solution’s concentration.


Concerning volumes: most of the time when working with such volumes in the lab, I would use measuring cylinders. These come with a rather poor precision but are adequate for most purposes. However, considering we need $\pu{300ml}$ and $\pu{900ml}$, we would probably want to work with very large cylinders so the error would be in the range of $5{-}\pu{10ml}$. It is possible to use much more precise equipment: in analytical lab courses, we would use pipettes that measure exactly $\pu{25.0 \pm 0.1 ml}$ and dilute using measuring flasks at a similar level of precision.

If, as it seems from the values you plugged, the initial solution was $\pu{4M}$ (1 significant figure), I would argue the result should be given to 1 significant figure too – introducing a large error. Arguably, a proper error propagation calculation would probably give you a better precision in the final result.