Div show/hide media query

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.

It sounds like you may be wanting to access the viewport of the device. You can do this by inserting this meta tag in your header.

<meta name="viewport" content="width=device-width, initial-scale=1.0">

http://www.w3schools.com/css/css_rwd_viewport.asp