def empty_columns(board):
    """Returns list of "" with length equal to length of expected column.
    >>> empty_columns(['***21**', '412453*', '423145*', '*543215', '*35214*',\
 '*41532*', '*2*1***'])
    ['', '', '', '', '', '', '']
    """
    columns = []
    while len(columns)!= len(board[0]):
        columns.append("")
    return columns