Remove sidebar and change width of primary div

I want to remove the sidebar from certain pages, so I’ve created a page template for this (and any other) ‘no sidebar’ pages. The template works – the sidebar is gone, but the primary div doesn’t re-size to fill the gap left by the sidebar. I see in the page template you have …

div id="primary" class="content-area <?php echo esc_attr( tdmacro_get_blog_primary_class() ); ?>">

…which sets the primary div with a class of .col-lg-8 (at full screen resolution). I need to change that so #primary will span 12 columns when the sidebar is missing. How can I make this change when the class name is assigned dynamically via the code above?

Thanks in advance!