HCHO OS for H is +1, O is -2 there are two H\'s one O on Carbon. 2 x (+1) + 1 x (- 2) = 0 thus, the OS of carbon is zero. CaH2 Ca is +2 thus, H is -1 HNO3 N is +5, O is -2 thus H + 5 + (-2) x 3 = 0 H = +1 P4 is an allotrope. All allotropes have an OS of zreo. Solution HCHO OS for H is +1, O is -2 there are two H\'s one O on Carbon. 2 x (+1) + 1 x (- 2) = 0 thus, the OS of carbon is zero. CaH2 Ca is +2 thus, H is -1 HNO3 N is +5, O is -2 thus H + 5 + (-2) x 3 = 0 H = +1 P4 is an allotrope. All allotropes have an OS of zreo..