I'm not sure, what you mean as the 'mobile width'. But in each case, the CSS @media can be used for hiding elements in the screen width basis. See some example:
<div id="my-content"></div>
...and:
@media screen and (min-width: 0px) and (max-width: 400px) {
#my-content { display: block; } /* show it on small screens */
}
@media screen and (min-width: 401px) and (max-width: 1024px) {
#my-content { display: none; } /* hide it elsewhere */
}
Some truly mobile detection is kind of hard programming and rather difficult. Eventually see the: http://detectmobilebrowsers.com/ or other similar sources.