Notes. Uncertainties are given in parentheses. ^(a)Sizes for which the new H₂ column densities are calculated as described in Sect. 3.8 of Bonfand et al. (2017). ^(b) H₂ column density calculated for the size indicated in the previous column. This value is used in Fig. 5b to plot chemical abundances relative to H₂. The uncertainties are calculated based on the uncertainties on the average H₂ column densities given in Table 1 and on the source sizes. ^(*) In the case of N3 we use the H₂ column density given in Table 1, measured in the 1 mm ALMA continuum map at 0.4′′ resolution (Table 1).