A vessel with a constant volume contains at a certain temperature an equilibrium mixture consisting of six moles of sulfur trioxide, four moles of nitrogen monoxide, one mole of nitrogen dioxide and two moles of sulfur dioxide. The reaction, sulfur dioxide + nitrogen dioxide ⇄ sulfur trioxide + nitrogen monoxide, is homogeneous in the gas phase. Four moles of nitrogen monoxide are introduced into the vessel. Calculate how many moles of the same substance there are when a new equilibrium is reached.