$opacity: ();
$opacity: map-merge(
        (
            '0':0,
            '25':25,
            '50':50,
            '70':70,
            '75':75,
            '80':80,
            '100':100,
        ),
        $opacity
);

@each $name, $val in $opacity {
    .opacity-#{$name} {
        opacity: #{$val / 100};
    }
}