Where did the acac come from in Cr(acac)₃ synthesis by CrCl₃ reagent? Those are the reactions:

$$\ce{(H2N)2CO + 3 H2O -&gt; CO2 + 2 NH4+ + 2OH-}$$
$$\ce{CH3COCH2COCH3 + OH- -&gt; acac- + H2O}$$
$$\ce{CrCl3.6H2O -&gt; Cr3+ + 3Cl- + 6H2O}$$
$$\ce{Cr3+ + 3acac- -&gt; Cr(acac)}3$$

I don't get from where comes the $\ce{C5H8O2}$ in reaction #2!
$\ce{CH3COCH2COCH3}$ is the condensed formula of pentane-2,4-dione, commonly known as acetylacetone. The $\ce{acac^-}$ is just a shorthand for the acetylacetonate ligand, i.e., $\ce{C5H7O2^-}$, which is just the deprotonated form of the acetylacetone used in reaction two.