There are 3 ways of doing that:
a)
...
'category_name' => 'category-1'
...
b)
...
'taxonomy' => 'category',
'term' => 'category-1',
...
c)
...
'tax_query' => array(
array( 'taxonomy' => 'category', 'field' => 'slug', 'terms' => array( 'category-1' ) )
)
...
More info: http://codex.wordpress.org/Function_Reference/WP_Query