Hydrogen iodide, hi, is formed in an equilibrium reaction when gaseous hydrogen and iodine gas are heated together. if 20.0 g of hydrogen and 20.0 g of iodine are heated, forming 10.0 g of hydrogen iodide, what mass of hydrogen remains unreacted?

19.92 grams

Explanation:

This questioned can be analysed on this table. All values that have been given in the question, are filled into the table as number of moles however, not as masses. I've written the values that were in the question in bold

                          H2            +           I2       >                 2HI

Initial (mol)       9.922                 0.0788                             0

change (mol)  -0.03909          -0.03909                       +0.07818

equil (mol)       9.88291               0.03971                        0.07818

As we can see 9.8829 mol of hydrogen remains unreacted. Converting that to mass:

mass(hydrogen)=mol*molar mass = 9.8829 mol * 2.01568

                                                        = 19.92 g