How to select nth parent p tag of div? [duplicate]

Am wondering how can we select 2nd p tag of a div.

html

<div id="my_div">
<p>
   <label>aaaa</label>
   <label>aaaa</label>
   <label>aaaa</label>
</p>
<p>
   <label>aaaa</label>
   <label>aaaa</label>
   <label>aaaa</label>
</p>
<p>
   <label>aaaa</label>
   <label>aaaa</label>
   <label>aaaa</label>
</p>

</div>

what i tried.

jQuery("#my_div :nth-child(2)").attr('style','width: 50%;');

this works but it also select p tag 2nd <label> but am only interested in parent p tag do not want to touch p tag childs. So any idea how to do it.