article {
    width: 100%;
}

@media all and (min-width:992px) {
    #envioForm > #formButtons > #btnEnviar{
        margin-right: 339px;
    }
}

@media all and (max-width:991px) {
    #envioForm > #formButtons > #btnEnviar{
        margin-right: 225px;
    }
}

@media all and (max-width:767px) {
    #envioForm > #formButtons > #btnEnviar{
       float: left;
    }
}