Okay, let's tackle this problem. The user wants a single-page web app using Tone.js that includes a pentatonic pad grid and a play/stop button with an arpeggiator. All sounds should be synthesized, no audio files.

First, I need to set up the HTML structure. The index.html will have a container for the pads and the play/stop button. I'll include the Tone.js library via a script tag since it's already provided.

Next, the CSS. I'll style the pads as clickable buttons, maybe using a grid layout. Each pad should have a unique class or data attribute to identify its note. The arpeggiator will need to cycle through these notes.

For the JavaScript, I'll initialize the Tone context and set up the synth with reverb. The synth should be a simple oscillator with some envelope and reverb effects. Then, create the pad grid. Each pad will trigger the synth when clicked, playing the corresponding note.

The arpeggiator part is a bit trickier. I'll need to create a sequence that cycles through the selected notes. The play/stop button will start and stop this sequence. The tempo should be adjustable, so I'll add a slider for that. The arpeggiator will use Tone.Transport to schedule events at the specified tempo.

Wait, the user mentioned a pentatonic scale. Let me choose a specific scale, like C major pentatonic (C, D, E, G, A). Each pad will correspond to these notes. When a pad is clicked, it triggers the synth with that note.

I need to make sure all the elements are connected. The pads' click events will activate the synth, and the arpeggiator will loop through them. Also, the play/stop button should control the transport.

Testing the code: Ensure that the Tone.js is loaded correctly, the pads work, and the arpeggiator runs smoothly. Adjust the tempo slider to see if the speed changes as expected.

Potential issues: Making sure the synth doesn't clash with the arpeggiator. Maybe use different oscillators or adjust the envelopes. Also, handling the reverb properly so the sound isn't too dry.

Putting it all together in one HTML file with inline CSS and JS. No external resources, so everything must be self-contained. Check for any syntax errors and ensure the Tone.js functions are called correctly.