$opacities: ( 0, 10, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, 100 );

.op\: {
    @include isMedia() using ($mc, $mb) {
        @include isList($opacities, $mc) using ($val) {
            opacity: opac($val);
        }
    }
}