Given an html5 application that I have access to, Is there a way to get a screen shot of how it would look on the iPhone 5.
I do not have an iPhone 5, so is this possible?
Sure - you can use a Mac for this.
Download Xcode and fire up the iOS Simulator and choose to emulate the iPhone 5. You will have a version of Mobile Safari that runs on OS X and will let you save screen shots of your app. Much of this is documented well by the PhoneGap steps listed for developing an iOS app.
Here I'm using skitch to grab a screen capture and I have reduced the size a bit to upload, but as long as your screen is large enough, you can capture a pixel perfect image easily. In this case, I was using Safari in the simulator and you'll need to follow the steps listed in the phonegap documentation to install Apache Cordoba so that your phonegap code can be seen by Xcode to push it over into the iOS simulator when you press Run in Xcode.
Image above from docs.phonegap.com