How to define variable in sass if/else statements? Is this possible?

How to define variable in sass if/else statements. Is this possible ? The code below gives the error that variable ch1 is undefined if i try to use it to create a color.

$opposite : 1; $oppositeCompl : 0;

@if $opposite == 1 { 
    $ch1 : 10;   
}

@if $oppositeCompl == 1 { 
    $ch1 : 50;
}

$c1 : hsla($ch1, 100%, 100%, 1);